A backdoor pilot is a type of pilot (yes, there are different types) that is filmed as a standalone episode so that it can still be aired on television even if it isn’t picked up.
If well-received, it can then be developed into a series or miniseries. It’s essentially a “test episode” for the intended series and marks an early step in development. A backdoor pilot may also be used for an episode that serves to introduce a spin-off.