DISEC refers to the United Nations General Assembly First Committee on Disarmament and International Security. It is featured at almost every MUN Conference, and is often one of the largest committees simulated at each conference.
What to Expect
Expect one of, if not the, largest committees you've ever seen. As such, very frequently teams will dump off their newbies/slackers in this committee while they allow more talented delegates to participate in smaller committees. However, due to the sheer size of DISEC, you will still be faced with a substantial number of formidable opponents. Expect many dual-delegations, which means that committees can be upwards of 300 people if most nations are represented and dual-delegated. This means that it will be necessary to learn twice as many names, deal with twice as many different personalities, and speak half as often as you would in another committee. Unmoderated becomes very important, especially forming coalitions of people, and managing your way through a crowd. Also, expect many different resolutions to emerge, and to spend enormous quantities of time merging, re-merging, debating, negotiating, and re-negotiating each of them. Expect everything to move slowly (often the entire first day of committee is devoted to setting the topic) and to only debate one topic. Most of all, though, expect one of the most exhilarating and fun MUN experiences you're ever going to have, 'cause there isn't much that beats 300 people in one unmoderated caucus shouting at each other about weapons exports.
DISEC is the GA First Committee for a reason: it deals with some of the most important and pressing issues of the day. As the name implies, you'll be dealing frequently with weapons and security issues, often Small Arms Proliferation, Nuclear Security, Missile Transfer Technology, Biological Weapons Proliferation, Chemical Weapons Proliferation (pretty much any weapons proliferation, in fact), and often specialized, regional topics such as Iran, North Korea, or other conflicted regions of the day.
The DISEC Website is very helpful (and often hard to find, as DISEC is more officially known and the UNFC, or UN First Committee). Also, Nuclear Threat Initiative is incredibly helpful for many topics you'll encounter in DISEC. The Acronym Institue for Disarmament Diplomacy is also incredibly useful.