CBSE Class 9 Answered

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Octet:---According to the rules of electronic configuration the last shell may contain maximum of eight electrons. The atom or ion containing eight electrons in its outermost orbit is exceptionally stable. This is octet.

The octet rule is a simple chemical  thumb rule of that states that atoms tend to combine in such a way that they each have eight electrons in their valence shells, giving them the same electronic configuration as a noble gas. This 8-electron configuration is especially stable because with 8 valence electrons, the s- and p-orbitals are completely filled (with 2 in the s-orbital, and 6 in the p-orbitals). Having completely filled orbitals provides increased stability due to something called "exchange energy."

So, basically molecules or ions tend to be most stable when the outermost electron shells of their constituent atoms contain eight electrons. The rule is commonly used in drawing Lewis dot structures