This is an entry in our Disneyland 101 Glossary: Disneyland Resort definitions in under 101 words, from Mouse Brief.
Wait time is the projected amount of time, in minutes, that guests entering an attraction queue will spend in that particular queue. Wait times are typically posted on signs at the entrance to each attraction as well as on “tip boards” in the Disneyland app and in the parks. Wait times are estimates and subject to surges in guest interest, maintenance issues and other factors. When you enter a queue, a cast member may occasionally ask you to carry a lanyard with a red card through the queue to the boarding area: this process helps cast members track wait times.