TNT (Tri-Nitro-Toluene) is a destructive block that, once activated, creates an explosion that can destroy surrounding blocks within a certain radius. It is originally a vanilla block but has a few crafting uses in Tekkit.
TNT is usually recommended as a quick and efficient method for destroying large portions of undesirable blocks or structures. It is also useful for clearing space for large builds, however it must be controlled properly. It can also be used as a weapon, but isn't recommended due to its destructive nature and its potential to seriously injure or even kill the player if they are caught within the blast radius.
TNT can also be used to craft Industrial TNT, however, you should note that Industrial TNT will create smaller explosion than original TNT.