If your askig this question you may have issues that the pump is set up for a valve system that you do not have. If the pump will not work to inflate your tire and you cannot get the pump to properly seat on the valve this is probably your problem.

When a mini pump is purchased it is normally set up for a schrader valve(car tire). The opening should be large and you set the pump onto the valve flip the lever behind the opening to lock the pump into place on the valve and pump up the tire.

If you have presta valves(skinny metal valve) the initial set up will not work. To change the pump to a presta valve set up unscrew the top portion above where you insert the valve. Flick the lever behind the valve and a small rubber piece should extend slightly, pull this out and then turn the pump over and smack it into your hand. Another plastic piece should fall out of the pump. Invert both of these pieces so that the plastic piece has the long pointed end facing down with the small opening facing up and the rubber piece is put back in with the small hole facing up. Screw the top back on, then unscrew the top metal piece of the presta valve. Tap it once to make sure the seal is broken and push the pump on, lock the lever in place and pump up the tire.