T-Mobile's parent company Deutsche Telekom (DT) has sold rights to 7,200 of it's US cell towers. The idea here is that the company who is buying the rights to the towers will be able to then lease space on those towers to other companies (think: AT&T, Verizon, and others) to bring in income. For T-Mobile to manage that would be a lot of extra work.
So, it's easier for T-Mobile to sell the rights to these 7,200 towers, which is a small portion of their 51,000 total, to a company that only manages leasing tower space to companies. Part of the $2.4 billion will be used to pay off some of DT's debt and some of it will be used to fund T-Mobile's network upgrades and LTE rollout.
Crown Castle, the company who is buying the rights to these towers, will own and/or manage 30,000 towers in 50 U.S. cities after the deal is complete. Crown Castle will have the option to buy the towers after about 28 years. For now, they can only manage them. T-Mobile will be a tenant at those towers for a minimum of 10 years.