Friction is essentially what you experience everyday. You can think of it as a “sticky” force I guess. It’s an interaction among atoms. Really, friction itself comes from gravity. Gravity pushes us towards the earth and this creates a normal force. If you calculate the friction coefficient, which is done in labs, you can multiply that coefficient by the normal force to get the force of friction. It always opposes the attempted motion. So you want to subtract it from your force in which is causing an acceleration to the object.