what is 'straight-and-level flight'

here is the sentence: Straight-and-level flight may be sustained at speeds fromvery slow to very fast.

could you please explain this technology term for me? thank you very much. :smiley:

It is not very technical, liqichao.

Straight: not curved; direct
Level: even or uniform height above ground
Sustained: maintained, kept, continued

For airplanes, straight and level flight means it flies straight (no yaw) without banking or pitching.