A pod is an obstacle that is designed to release a weapon that can range from a bomb, to random traps, 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 and a mislabeled barbed wire net that Mitchell was pushed onto by Peeta in his rage which caused chains to wrap around Mitchell's arms and legs and a barbed wire netting to wrap around, hoisting him into the air leaving him completely immobile. Some pods were much more terrifying, like the energy beam which caused 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. There was one certain pod found in the Transfer, which was found by Squad 451 when the floor collapsed; a grouping of moving blades called a meat grinder, which would mutilate any victim beyond recognition and is thought to have killed Jackson and or Leeg 1.
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 which caused all the people on a certain patch of street, both Capitol citizen and soldier, to bleed uncontrollably to death from every orifice. There was also a pod which released a huge burst of steam which boiled everyone and everything in its way. These pods are very much like the engineered traps in the Hunger Games arenas. They resemble a huge, final Hunger Games, where President Snow would be a player, too.