Resiliency is a quality in objects to hold or recover their shape, or in people to stay intact. This is a kind of strength.
If you bend a fork and it bends right back — that's resiliency. A car that is in an accident only has a few scratches has resiliency: it holds up and keeps its shape. A car that was demolished didn't have much resiliency. A person shows resiliency if they are able to recover from a difficult experience. Going back to school after a horrible event like a parent dying shows a lot of resiliency. Doing a triathlon shows physical resiliency.