Organized Team Activities, as the name suggests, is the official NFL term used to describe team get togethers outside of the traditional season. All teams hold a mini-training camp in the spring to work on player development, skills, and introduce new players to the team systems and schemes. OTAs can also refer to other get togethers outside of football such as charity events.
The collective bargaining agreement with the NFLPA states that OTAs are not mandatory and limits the number of OTAs a team is allowed to have. Most players do attend, but occasionally a disgruntled player may decide to skip until his contract is renegotiated.