Question

what do you mean by octet in chemistry

Answer 1

when the outermost shell of an atom contains all 8 electrons then the atom is said to attain octet.

hope this helps you.

Answer 2

An octet is a valence shell having 8 or multiple of 8 electrons. This number provides stability to the atoms and is also the reason atoms react which each other. they tend to give or take electrons to complete their valence shells.
One exception (Helium is the Mongol of Chemistry XD),
Helium is a stable atom with only 2 electron in its valence shell. It is a ducet atom