Force is a push or a pull that acts on an object.  For example, when you throw a baseball you give the baseball a force.  Newton's 2nd law states that if you give an object force, the object accelerates (in the same direction as the force you gave to the object).  Thus, when you throw a baseball, the baseball accelerates in the same direction as the force you gave to it.
 

Back to Newton's Three Laws of Motion.