What are leaving groups in nucleophilic substitution reactions?
The leaving group is the part of the substrate that is missing at the end of the reaction.
The equation for a typical nucleophilic substitution reaction is
Nu⁻ + R-L → Nu-R + L⁻
Nu⁻ is the nucleophile, and R-L is the substrate.
The L group is missing from the substrate, so L is the leaving group.
Suppose the reaction had been
N≡C⁻ + CH₃CH₃CH₂-Br → CH₃CH₂CH₂-C≡N + Br⁻
The product no longer has the Br atom that was in the CH₃CH₃CH₂-Br.
The leaving group was the Br atom.
Here's a video on leaving groups.