What is this "shadow" called and what's the purpose?

Question

When you slide Chrome browser (or FireFox) to extreme left, right or upwards, you see this shadow (for want of a better word) for a few seconds, as shown



enter image description here
enter image description here



What is it called and what's the purpose ? (Can't Google since I don't know what it is :)



It's on Moto X Play and it's not the jelly scrolling reported on some recent phones


— beeshyams | Sources

Answer

— Firelord | Sources

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