for and isspace

Hello! I have a question about for loops and isspace.

for (i = 0; isspace(s[i]); i++)

In this for loop I don't understand how it knows when to stop based on the isspace(s[i]). I thought the second condition had to be something like "until such and such happens the loop continues" but I don't see that happening here.
Thanks!

until current character is whitespace - do something

ohhh ok, thank you!