Trample is a keyword ability that lets a creature's combat damage spill over. When a creature with trample is blocked, you only need to assign enough damage to the blocking creature to destroy it. Any extra damage can be dealt to the player or planeswalker the creature is attacking, instead of being wasted. The reminder text puts it simply: the creature can deal excess combat damage to the player it's attacking. Some spells can also have trample, letting them deal excess damage to the target's controller.
In play, trample makes large creatures difficult to stop with small blockers. A tiny creature can't just absorb all the damage—the rest still gets through to the player. That forces opponents to find bigger blockers or chump block less effectively, and it makes trample a staple of aggressive red and green decks. It turns even a single big attacker into a serious threat.
