There are lots of ways to conceptualize how we move. One of those ways involves describing whether a motion is open chain or closed chain. (There’s also pseudo-closed chain, but I’ll leave that one out for now.)
A closed chain movement means that the body’s path through the motion is restricted in some way, say with a pushup. The arms are against the ground and are not able to move in that direction.
A bench press is the same motion, but with the arms free to move in space rather than stabilized against the ground. It is an open chain activity.
As a rule, open chain movements provide more challenge for the body because we have to use our stabilizing systems to control things rather than some other barrier. (All weights being equal, that is.)
Explore the exercises in your next workout: which ones are open? Which ones are closed? Can you think of an exercise that works the same muscle groups with the other chain option?
Go play.

