Utilization is a calculation of how busy your team is with client work, calculated by a % of time you're working with a client.
Utilization = Billable Time / Available Time
Most agencies will shoot for a utilization rate of ~85% for most roles. Meaning, staff is doing billable, client work for 85% of their time. Anything below that, and you may not be covering their salary with client revenue. Above that and there isn't room for casual collaboration, space, and sanity.