A cap hold is an amount charged against a team's cap for a player it does not have under contract yet, usually a free agent it still holds rights to, so the team cannot spend the same room twice.
In HoopSim there are three kinds of cap hold, all from restricted free agency: a qualifying offer you have issued (the QO amount), a qualifying-offer decision still open on a player whose deal just ended (the QO amount, in place of their old salary), and an offer sheet you sent to another team's restricted free agent that is waiting on their answer (its first-year salary). Nothing else is a hold: Bird rights on your other free agents cost no cap room, which is simpler than the real rules. Holds count in your total committed money for the cap, the aprons and the tax.
Rulebook: Payroll & holds. Related: qualifying offer, restricted free agent, cap space.