while wend

quick question
Ive been looking around online but still have not gotten a good grasp here.

WHILE LEN(INKEY$): WEND

I understand the use of WHILE/WEND, LEN, and INKEY$. But i am having trouble understanding exactly why this works.

It would seem like there would need to be another variable there.
example.

WHILE LEN(INKEY$)= X: WEND