I've been asked about this several times, so I'll explain it here.
CVX holders can lock their CVX as vlCVX (for at least 16 weeks) to vote on how the veCRV controlled by Convex is used.
Convex pools scattered CRV and locks it permanently, so users who don't want to lock their own tokens for four years can still indirectly earn boosted rewards.
$CRV is Curve's governance token, and Curve popularized the veToken model.
Anything to add? Let's talk in the comments.