A Collider is an object that defines a hit area that triggers a collision event in your game. For example, you might create a Collider on a character's sword and another collider on an enemy's torso. You can then tell your game engine to subtract hit points from the enemy whenever the two objects collide.
You can find the Create Colliders tool under the Create Tools flyout menu (or use the keyboard shortcut
Change Collider Shape
Once you've created a Collider, you can change its properties in the Selection Panel under Collider Options. Choose Polygon under the shape dropdown to create a custom shape.
One particularly useful property of Colliders is that you can animate their Enabled state (true or false). If your character has a lightsaber, you'd want to make sure your collider is enabled only when the lightsaber is turned on. If your character has equipped a shield, you might want the shield's collider to be disabled during a rest/idle pose, but enabled when holding a guard/shield pose.