For example: https://www.reddit.com/r/HadToHurt/comments/btfdu4/hot_oil_poured_down_the_back_of_my_arm/?utm_source=share&utm_medium=ios_app
A bad sunburn or other burn can cause some damage to the dermis, the living layer of cells below the epidermis. When this happen, the blood vessels will start to get leaky there to fill it up with plasma, which cushions and protects the epidermis as it heals. The plasma in it is basically just blood without the big components like cells (except for white blood cells that’ll crawl out on their own).
If it gets infected, foreign cells can reproduce in abundance, causing white blood cells to also go there in abundance, at which point the fluid is called puss.
The fluid is usually either blood plasma, which is the stuff that makes up your blood that isn’t a white or red blood cell, or Serum which is the same thing as plasma, but without any of the clotting factor that plasma has.
Some blisters can become filled with blood directly after injuries. They have the sadly boring name “blood blisters.”
It is essential a biological burn bandage. The fluid is sterile and the bubble keeps everything else out. Burns need to replace the damaged skin which is what keeps bad stuff out. So the bubble and liquid makes a clean environment to regrow the burned skin. Now that that’s taken care of. Do not pop blisters unless absolutely necessary. If you or it pops by itself do not remove the skin bubble. Without the liquid it is not sterile but the skin flap is much better than nothing just put a bandaid or bandage over it.