Lockdown refers to a specific style of melee build. The focus isn't on damage, but instead control. The general principle of it is to threaten as large of an area as possible with your weapon and then trip enemies inside of it. Doing so allows you to more readily control the positioning on the battlefield.

Say you threaten 10ft with a polearm and an enemies moves through this area provoking an attack of opportunity. You substitute a trip attempt for the attack, and if you succeed the enemy is now prone on the ground. His movement has stopped and the rest of his turn is wasted. He even has to spend his next move action to stand up.

Another possibility is that the enemies having seen this, will waste actions trying to go around your area of control. This accomplishes the same goal, by letting you and your allies dictate battlefield placement.