A pod is an obstacle that is designed to release a weapon that can range from a bomb to a group of mutts. Whatever is in the pod is designed to trap or kill whoever activated it or whoever is in proximity of the detonation.
The Capitol contains and creates many pods, some that have been there since the Dark Days and others that were added over the years. Plutarch Heavensbee designed a number of them himself. New pods were put in after he fled the Capitol with a Holo map of the pods' locations to assist the rebels.
Some of the pods mentioned in Mockingjay are the explosion that killed Boggs, a barbed wire net that Mitchell was launched into by Peeta in his rage, a sort of beam that cause Messalla's flesh to melt off his body, a large tidal wave of a black substance, and a swarm of tracker jackers. There are also pods that release gas, bullets, mutts, and metal darts that killed Leeg 2 in Mockingjay.
Notably, the pod which is set off near Snow's Mansion, where the entire street hears a cracking sound, and the pavement begins to fold in on itself and collapse away, allowed the citizens and rebels to fall into the pit of mutts below. Another pod which was a purple beam of light was used for the same purpose. There was also a pod which released a huge burst of steam which boiled everyone and everything in its way.