Quorum is usually expressed as a share of total supply that must vote, for example 4%. It exists to stop a handful of tokens deciding a protocol's future during a quiet week.
Low quorums have been exploited. An attacker who can borrow or flash-borrow enough voting power for one block can pass a proposal that sends the treasury to themselves, which is why snapshot-based voting power measured at an earlier block is standard defence. See flash-loan-attack.
High quorums have the opposite failure: genuine apathy means necessary fixes stall, and the protocol becomes dependent on a few large delegates showing up. Most mature systems solve this with delegation, which concentrates power in a small number of professional voters and is honest about it rather than pretending otherwise.
Related: governance-proposal, dao, governance-token, vote-escrow