A slack adjuster transfers force from the brake chamber pushrod to turn the brake camshaft. Because brake linings get thinner as you use them, the gap between the shoes and drum grows wider. This would require the pushrod to travel further to apply the brakes. The slack adjuster has an internal gear mechanism that compensates for this. It takes up the "slack" by rotating the camshaft slightly, keeping the pushrod stroke within safe limits.
Almost all modern trucks use automatic slack adjusters (ASAs). They measure the pushrod stroke during braking and adjust the gear position automatically. Never manually adjust a slack adjuster that is backing off; manual adjustments mask a mechanical failure in the adjuster gears or worn camshaft bushings that must be replaced.