What happened during the other interglacial periods?
It’s difficult to answer without a more specific question about what happened (perhaps in relation to a particular type of life, or the oceans, or what was happening in a certain region, etc.)
Here is an answer I gave on r/askscience a little while ago to the question “Are there any major differences between the last couple of interglacials?”, though I wouldn’t say it’s particularly Eli5 friendly:
Well, just looking at [this handy chart for marine isotope stages](http://railsback.org/Fundamentals/SFMGSubstages01.jpg), we can see that the last few interglacials arguably include to some extent some substages of MIS 7 (7a, 7c and 7e) and MIS 9e in addition to the ones mentioned in your title. This notation of letters for MIS substages has been in use for a while, but the chart shown is the most recent effort to standardise them all into a consistent scheme published in 2015,^(1) I’ll be sticking with this particular arrangement as it is up to date and sorts out some previous discrepancies that were going on with MIS 9.
Of these most recent interglacials associated with various substages of MIS 5, 7, 9 and 11 then, 7 sticks out as being more fragmented and not as significant as the others, which are all in line with the Mid-Brunhes shift to more extreme interglacials shortly before MIS 11 (The Brunhes Chron is the current period of normal polarity in magnetostratigraphy, extending back to MIS 19c at about 781 kya).
Whether just MIS 7e is designated as the interglacial here or 7e, 7c and 7a rests upon how we define interglacials at all – a sea level definition encompasses all whilst orbital models based on the 100kya Milankovitch cycle allows for only 7e. I think it’s a bit cavalier to start moulding reality to fit certain model expectancies so I prefer the former. More discussion on this and the various pros and cons of how we are to define interglacials at all can be found in the first few pages of a comprehensive review paper on the subject published in 2016.^(2) I’m also inclined to include the 7a and 7c substages as they are centred around 200 kya which correlates with the so called Aveley Interglacial of Europe, taking its name from a small town in southern England in which important Neanderthal remains have been found from the time (along with both elephants and mammoths in the area – what a time to be alive!). In terms of North America, the correlation of MIS 7 to lithological strata is a bit more ropey, but I believe the top of an extensive paleosol unit (the Yarmouth-Sangamon Paleosol across the US Midwest) was formed at the same time, which would mean there really was a withdrawal of ice-sheets across both North American and European continents.
Ok, so having dealt with the slightly different interglacial of MIS 7, lets look more briefly at the others. Concerning the most recent interglacial it is specifically MIS 5e that is relevant, correlating with the Eemian Stage in America, known as the Ipswichian in the UK. The other MIS 5 substages are merely subsequent stadials/interstadials. MIS 5e is about 15,000 years long, MIS 9e is about 18,000 years long and MIS 11c about 22,000 years. So they are all of a similar length, though 11c is clearly the longest – it has also been called the warmest interglacial of the last half a million years, with sea level at least 6 m above present levels based on other evidence including plankton ecology, reef building trends and North Atlantic Deep Water production inferred from carbon isotope data.^(3) As far as the MIS oxygen isotope stratigraphy goes though, stages 1, 5e, 9e and 11c are all extremely similar in extent.
(1) [Railsback, L. B., Gibbard, P. L., Head, M. J., Voarintsoa, N. R. G., and Toucanne, S. (2015), An optimized scheme of lettered marine isotope substages for the last 1.0 million years, and the climatostratigraphic nature of isotope stages and substages, Quat. Sci. Rev., 111, 94– 106](https://www.sciencedirect.com/science/article/abs/pii/S0277379115000360?via%3Dihub)
(2) [Past Interglacials Working Group of PAGES (2016), Interglacials of the Last 800,000 years, Rev. Geophys., 54, 162-219](https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1002/2015RG000482)
(3) [Howard, William R., ‘A Warm Future in the Past’, Nature 388, 418-419 (1997), Carbonate Marine System During Oxygen Isotope Stage 11, American Geophysical Union Spring Meeting, Baltimore, Maryland, USA, 27-30 May 1997](https://www.nature.com/articles/41201)
Can you clarify your question? You are asking about what happened across the entire planet during numerous spans of thousands of years. A lot of stuff happened.