。
Don't move from there until I give the signal.
Literal
I [subject-が] signal do until there [object-を] don't-move [so-as-to-ように].
ように at the end of a command transforms it into a softer, more considered directive — especially common when giving instructions. 動かない is the negative of 動く ('to move'). そこを — the を here with a motion verb marks 'from there' (path/location departed from). 合図する ('to signal') is a noun+する verb.