Penal Code 1473.7 lets a person who is no longer in criminal custody file a motion to vacate a conviction or sentence. "Vacate" simply means to cancel or set aside. If the court grants the motion, the conviction is treated as legally invalid, and in most cases the person is allowed to withdraw the guilty or no-contest plea they originally entered.
The law took effect on January 1, 2017. It was created by Assembly Bill 813, which added this section to the Penal Code to give people a remedy they did not have before.
A quick note on the numbers, since they look similar. Penal Code 1473 is California's general habeas corpus statute, the traditional path for challenging a conviction. Penal Code 1473.7 is the newer, related provision built specifically for people who are out of custody. The difference matters, and it is the whole reason this law exists.