In short: The FCC wrote a letter to Sprint and T-Mobile on Tuesday, stating that it was pausing the informal 180-day clock for reviewing mergers, as it needs more time to review the proposed deal. This would allow third-parties to review it, as well as allowing for a thorough review of staff at the FCC. This is because the two companies recently submitted new materials to the FCC and it needs time to go through those materials before approving this deal.
Background: Sprint and T-Mobile have been off and on in regards to combining into one carrier for quite a few years. Under the Obama Administration, it was clear that the deal would not get approved. So both parties had waited until a new President took office, which happened in 2017. Then the two parties decided to get back involved in talks to merge as one carrier. In April of this year, the merger was made official, and sent to the FCC as well as the Department of Justice for review. T-Mobile expects the merger to close next year, in which case, T-Mobile would absorb Sprint, with T-Mobile's CEO John Legere being in charge of the combined company. However, this all depends on the FCC approving the deal.
The impact: This could be a good thing or a bad thing for this potential deal. It could allow the FCC time to look through the deal more thorough and decide that the deal wouldn't be a good thing for Americans, or it could decide that it is a good thing. T-Mobile is still trying to get this approved, and the Ericsson deal it announced today for 5G was one of the parts of helping to get it approved. By showing that it is really working to get a 5G network up and going.