The intercostal muscles are a group of muscles found in between the ribs. They allow the ribcage to move during ventilation (breathing in and out).
The mussel (muscle) is really into costumes (intercostal) and it hangs all its costumes on the ribs.
When you inhale, the external intercostal muscles contract and the internal intercostal muscles expand the ribcage, moving it up and outwards. When you exhale, the external intercostal muscles relax and the internal ones contract, moving the ribcage inwards and downwards.