It's a literal description of how we walk.
Here's why it's not an idiom:
* Literal meaning: The phrase directly describes the physical action of walking.
* Figurative meaning: There's no metaphorical or symbolic meaning associated with it.
* Idioms have figurative meanings: Idioms are phrases with meanings that are different from the literal meanings of the words they contain.
Examples of idioms:
* "Kick the bucket" means to die.
* "Spill the beans" means to reveal a secret.
While "one foot in front of the other" isn't an idiom, it can be used figuratively to mean "taking things slowly and steadily," or "making progress gradually." But even in this case, it's more of a metaphor than a true idiom.