Eww! Did you step in poop! Yuck!
Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means step and the verb vocab version is to step on. So, not quite "to step" like you might expect, but at least you're stepping still, though it's on something.
You want to step on that bug? You fool (ふ)! That's a killer shoe stabbing bug, it will cut right through your sole and poison you via your foot! Fool!