Last updated for Star Citizen 2.6.3
REC is a currency in Star Citizen which can currently be used to rent ships and weapons. You can earn REC in-game for free and spend it on the webstore.
REC can be earned by participating in games inside the Arena Commander module. Upon completion of the match, you'll be awarded REC depending on how well you performed.
For example in the deathmatch gamemode, the more kills you get, the more REC you'll earn!
The amount of REC you have can be seen by visiting and signing in to any page on RobertsSpaceIndustries.com, and then clicking the 'My RSI' button at the top of any page.
You can spend your REC by visiting the Electronic Access store. In the EA store, you can rent ships and weapons to use in-game. (As of 3.0 rented ships can only be used in your hanger and Arena Commander. Weapons, however, can be used in any game-mode)
Once you've rented an item with REC you won't have access to it forever. You'll be able to use it for 7, 24 hour periods.
This means that if you use the item every day for a week, then it will disappear from your account exactly 7 days after you purchased it, however, if you only use the item once every week, it'll disappear 7 weeks after your purchase.