We've already learned about the baserunner and how they are trying to become safe on-base. In order to be called safe by an umpire, the baserunner needs to touch the bag with a part of his body. Sometimes, running towards a base just won't cut it.
Sliding is a technique that baserunners use in order to avoid being tagged out with the baseball. When a baserunner approaches a base, and a fielder near him has the baseball, the runner will drop to the ground and -- using his momentum from running -- slide feet-first into the base. Although sliding seems counterintuitive since it actually is slower than running, it is harder for fielders to tag a runner who is sliding.
Sliding to 1st Base
Sometimes, a player will have too much momentum or begin his slide too late, therefore sliding too far or too fast past the base and not making contact with it. In this situation, it is more likely that the fielders will be able to tag him out.
PRO TIP: A batter-runner cannot be tagged out if he overruns or overslides on first base. However, this is not the case if the player is a baserunner. If the batter-runner chooses to run towards second base, he can be tagged out.