Absurdism is Camus's name for the collision between two things that won't go away: the human instinct to seek meaning, purpose, and order — and a universe that, as far as we can tell, supplies none of those things on its own. Most philosophies try to resolve the collision. Camus refused.
His view: don't pretend the universe has meaning (that's "philosophical suicide" — the kind of escape religion or fixed ideologies offer). Don't kill yourself either (that's literal suicide, an escape too). Instead, hold the absurd in view, and live anyway. The myth of Sisyphus — eternally pushing a boulder up a hill only to watch it roll down — becomes a model: "one must imagine Sisyphus happy" because his rebellion against the meaninglessness is itself the meaning.
Absurdism overlaps with existentialism but is more austere. Existentialists build meaning through commitment. Absurdists hold the question open. The strangeness of being conscious in an indifferent cosmos isn't a problem to solve — it's the condition we live with.
In practice, absurdism looks a lot like comedy. The cosmos doesn't care that you exist; here's a tomato sandwich anyway.