The most important point: This is for asymptomatic cases. After that five-day period of isolation, those who have tested positive should continue to wear a mask when around others for another five days afterward.
In another change, those who have been exposed to an infected person can now forgo self-quarantine only if they have received a COVID booster and self-mask around others for 10 days after that exposure. If you haven’t received your booster yet, you don’t count as “fully” vaccinated under those guidelines.
The reason for the changing guidance is, as always, new data. The CDC says that those who have tested positive for COVID but who are feeling no significant symptoms tend to be contagious for only a short period of time—and given the likelihood that many of those infected were contagious a day or two before they managed to get tested, a five-day period still grants a few days of leeway. While some skeptics have expressed worry that the shortened isolation time presents risks, the data generally supports a reduced isolation period. In the rarer cases in which a person remains contagious longer than those five days, strict adherence to masking should mitigate what risk remains.