By default, each project on the protocol pays a 2.5% fee of outbound funds down to $NANA (project #1 on the V4 protocol). If the project's ruleset has the holdFees flag on, this 2.5% will be set aside (or, "held") but not processed into $NANA. If you then put funds back into your juicebox, the fees being held can be returned to your treasury.
Held fees are useful for projects that need to use raised funds to buy something, a copy of the U.S. Constitution for example, but might fail and then need to return funds to contributors. In such cases, contributors can be made whole – Juicebox DAO only participates in successful projects.
After 28 days, any held fees can be processed. If a project desires holding fees for longer, they must add the funds to the project and then withdraw again – like a "proof-of-funds".