Rolling reserves let the processor protect itself if you start having chargebacks. Typically the processor holds 5%–10% of every batch for 90–180 days, releasing the funds back to you on a rolling schedule (so day 91, you start receiving day 1's reserve back).
Reserves are common on high-risk MCCs (smoke shops, supplements, coaching, anything with high refund rates), on new accounts that haven't built history, and on accounts with sudden volume spikes that don't match the application's stated volume. They're usually negotiable down (or off) once you've built a clean payment history.
The cash-flow impact can be severe. A $50,000/month merchant with a 10% / 180-day reserve has $30,000 sitting unaccessible at any given time.