All molecules have kinetic energy (energy of movement), this causes the molecules to move around randomly.
To visualise this, imagine a room that has been cleared of furniture. Into the centre of this room introduce a small radio controlled toy vehicle that once it has been turned on will move off in one direction and will only change direction when it collides with something.
The new direction it takes will depend on the angle it hits wall at. If left operating this small vehicle will continue to move about the room bumping into walls and bouncing off to move in a new direction.
Few molecules exist in isolation, usually there are millions of molecules, all moving around like the small toy car in the earlier visualisation.
Obviously collisions with other molecules will be common but how common?