Quorum
The minimum amount of voting power that must take part for a governance vote to count as valid.
A quorum is expressed either as an absolute number of votes or as a percentage of circulating or total supply, and it exists so a small group cannot pass changes while everyone else is inattentive. Set too high, proposals fail repeatedly for lack of participation, which is the usual driver behind formal delegate systems; set too low, capturing the protocol becomes cheap. Implementations differ over whether abstentions and votes against count toward the threshold or only votes in favor, which changes how easy it is to block a proposal by not voting. A proposal that misses quorum is not rejected on the merits, it is simply undecided.
In practice
A protocol whose quorum requires a percentage of supply to vote may see proposals expire unvoted, prompting a governance change to lower the threshold or formalize delegation.
The common misunderstanding
Failing to reach quorum is not the same as being voted down; the proposal simply did not attract enough participation to be decided.