If the Chairman of the 1922 committee receives 48 letters from Conservative MPs who have lost confidence in their leader he must decide whether to hold a vote of no confidence.
When that happens, the Prime Minister has to receive more than half the votes from her MPs if she is to remain in power. If she gets those votes, she carries on, and there cannot be another vote for at least a year.
If she doesn't get the required number of votes, she must resign immediately, and that triggers a contest for the next leader of the party.
So to sum up, Theresa May remains as Prime Minister until there has been a vote by her MPs. If she loses the vote, she loses her job.