Floo? It should* read "flue" network. Flues are quite narrow, so only a very small child would pass through them without difficulty. Older boys like Ron and Harry would need to "tuck in their elbows", so as not to get stuck.
To make a "tuck" is to make a fold, so the idea is to fold your arms (probably across your chest, as though you are hugging yourself) - in such a way that your elbows don't stick out.
Or it may mean "keep your elbows in against your sides, as TheEnglishCondor says.
👁 Smile :)
(*In what we like to think of as the real world.)
