A modex is part of the Aircraft Visual Identification System* which consists of either one or two letters used by Dept of Navy (USN and USMC) aircraft to identify aircraft unit assignment and the aircraft side numbers (as assigned below).
The first letter of a modex indicates whether the organization (wing, group, squadron, station) owning the aircraft originated with Marine/Naval Air Forces Atlantic (A through M), Marine/Naval Air Forces Pacific (N through Z) or in the case of CNATRA a single letter (A through G) for the Training Wing.
In carrier air wings the first digit of a side number indicates an individual aircraft squadron in the air wing.
1xx and 2xx side numbers were assigned to fighter squadrons (VF) until the F-14 Tomcat was retired, they are now assigned to strike fighter squadrons (VFA) flying the F/A-18E or F/A-18F Super Hornet.
3xx and 4xx side numbers belonged to light attack aircraft squadrons (VA) flying the A-7 Corsair until the retirement of those aircraft when they were replaced by Strike Fighter Squadrons (VFA) flying the F/A-18C Hornet.
5xx side numbers were reserved for heavy attack aircraft squadrons (VA) flying the A-6 Intruder until the A-6 was retired. The 5xx side numbers are now assigned to the Electronic Attack (VAQ) Squadron (EA-6B/EA-18G).
6xx side numbers are assigned to the Airborne Early Warning(VAW) squadron (600-605) flying the E-2C, and the Helicopter Sea Combat (HSC) squadron (610 and up) flying the MH-60S helicopter.
7xx side numbers were assigned to the VS squadron flying the S-3 until the dissestablishment of those squadrons. 7xx side numbers are now assigned to the Helicopter Maritime Strike (HSM) squadron flying the MH-60R helicopter.
The last two digits identify an individual aircraft within a squadron with the aircraft numbered sequentially from X00 (or 610 in the case of the HSC squadron) up the to the number of aircraft in that squadron with the number X13 often skipped. Some squadrons may skip numbers other than X13 if past mishaps resulted in the loss of an aircraft or crew with that number. Each squadron will typically designate its lowest numbered aircraft as the CAG's aircraft (called the CAG bird) and some will paint its tail with squadron colors rather than with the subdued grey tone colors of the rest of the squadron's aircraft.
Marine aircraft typically use two-digit side numbers that do not include squadron information. If USMC aircraft are assigned to a carrier air wing they employ the three-digit number system used by the rest of the carrier air wing.
Land-based naval aircraft may use three-digit side numbers, but they typically are the last three digits of the aircraft's bureau number (BUNO) and do not give any information concerning the aircraft's squadron.
Carrier onboard delivery aircraft also used BUNO derived side numbers, but occasionally use the aircraft carrier's number (for example, 68 for an aircraft assigned to USS Nimitz: CVN-68).
You can find some additional information on Navy aircraft serial history at
http://www.joebaugher.com/navy_serials/navyserials.html
