A Push Composition involves a team based around heroes that can push towers and objectives relatively quickly. A typical push comp looks like this:
This particular comp has three main push heroes (Bio, Gara, and MK) while also having two main AD carries (Kerrigan and Nova) for DPS to win teamfights. The Marine King aura helps these two heroes push hard as well as helping the rest of the team push. The idea is that one teamfight should lead to several objectives being cleared for this team due to their incredible push power. This will snowball the team to victory.