is this from the verb PASSARE? doesn't that mean ''to pass by''??
Yes, it's for the verb passare, but it doesn't only mean to pass by, also "to pass to" as in this case, or simply "to pass" (e.g. il tempo passa - time passes).
Why didn't this accept "Pass the plate to the boy," which would be said like a command?
"Passo" is the conjugation for "I pass", so it can't be a command; for that it'd have to be conjugated to a second person and imperative (passa, passate, or formal passi, passino).