What is that dx thingy at the end of an integral

Question

Some of my professors claim it’s the infinitismal section of the graph, and some say it’s just notation. Then my professor said the differential of a function and the derivative of a function are two different things, whatever that means. I’m just all very confused. Plz help.

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Answer 1

https://en.wikipedia.org/wiki/Integral#Terminology_and_notation

“The symbol dx, called the differential of the variable x, indicates that the variable of integration is x.”