Answer:
The phrase is a metaphor as running is a fun experience and can be associated with laughter.
Step-by-step explanation:
the phrase insinuates that the narrator is so free and happy in the poem's narrative that she is able to laugh from her legs. this is because she is absolutely free to run wherever she wants to run to and has no bounds hence the running becomes an enjoyable activity that one would do laughing.
A metaphor is when a word is when a word or a phrase gets associated with an action or an object with which it does not make sense and in the process derives new meaning. here, laughing from the feet is a clear example.