From HaloWaypoint.

Your Competitive Skill Rank (CSR) measures your performance against other players in Halo 4 Matchmaking War Games. You will have a CSR specific to each active playlist. Your best CSR is shown on your Player Card and Service Record, as well as Player Compare. Your Competitive Skill Rank can be seen on all screens of Halo Waypoint and measures your performance against other players in Halo 4 Matchmaking War Games. It is a number from 1 to 50 (higher is better) which will go up and down based on match outcomes. You will have a CSR specific to each active playlist, and your best CSR is shown on your Player Card and Service Record, as well as Player Compare.

Competitive Skill Rank is a brand new feature for Halo 4 and to celebrate, we’re issuing you a challenge: Play in the Castle DLC playlist from April 8th at 2am PST to April 22nd at 2 am PST, reach CSR level 35 or higher at any point on or between these dates, and you’ll receive an exclusive avatar tee!*

*To receive avatar item, you must play Halo 4 standard edition and achieve a CSR rank of 35 or higher on the Castle Team DLC Playlist while connected to Xbox LIVE, any time between April 8 2013, 2AM PST and April 22 2013, 2 AM PST. A download code will be sent to your Xbox LIVE messages inbox associated with your Xbox LIVE account by May 13, 2013; Limit one avatar item per gamertag. Xbox LIVE Gold membership required for Halo Infinity Multiplayer (sold separately). Offer available to users 17+, in all Xbox LIVE countries except Italy, and is non-transferable, not redeemable for cash, and cannot be combined with any other rebate or offer. In New Zealand, only adult accounts (18+) can download or stream content over the Xbox LIVE marketplace. Taxes, if any, are the sole responsibility of the recipient. Download charges may apply as set by your ISP. For Xbox LIVE Terms of Use and system requirements, see www.xbox.com/live. Offer provided by: Microsoft Corporation, One Microsoft Way, Redmond, WA 98052.