Crash Landing is a relatively new keyword action (introduced in Magic's spacefaring set) that turns one of your face-up "spacecraft" or vehicle-style permanents back into its grounded state. In plain English: you take a permanent that was "in flight" (often tapped or in an active mode) and crash-land it, returning it to a basic, non-active form—usually as part of an ability's cost or trigger.
The nuance players miss is that crash landing is not the same as the permanent dying or leaving the battlefield. The card stays under your control; it just changes status. So effects that care about a permanent leaving play, or "dies" triggers, won't fire from crash landing alone.
In Commander, it matters for tempo and resetting key permanents—you sacrifice a flying threat's aggression to unlock a powerful grounded effect, then potentially relaunch later for repeated value.