。
Fill it up, please.
Literal
Full-tank [into-に] make-[polite request-てください].
A gas-station staple. 満タン ('full tank') combines the kanji 満 with the katakana タン (from English 'tank'), a common hybrid compound. The pattern [noun]にする means 'to make [it] into [noun]' — a state change request. The full phrase: 'please make it (the tank) into the full-tank state.'