It is the signal to perform a fuel check. (Actually: "How much fuel have you?")
The hand mimics drinking from a bottle, and when given by a formation flight lead it is intended to prompt the wingman to respond with a number of fingers to indicate fuel remaining in hundreds of pounds.
There are several pages of inter-flight signals in table form found in the NAVAIR 00-80T-113, "AIRCRAFT SIGNALS NATOPS MANUAL". (For US Navy, the USAF likely has a similar document...) A copy can be found here: SIGNALS NATOPS
This particular signal is found on page 1-8 and is reproduced below. (Note - the pinky seen in the photo isn't specified, but is generally used to ensure that the signal isn't confused with a closed fist)