These cases hold, basically, that if a defense lawyer messes up when representing a person in connection with a decision to plead guilty, that violates the person’s constitutional right to effective assistance of counsel. As a result, the harm that was done by the lawyer’s mistake can, and must, be undone. So, a person who was hurt by a lawyer’s mistake can challenge what happened to him and possibly have a conviction or sentence undone.
The New York Times took the notable step of issuing an editorial praising the decisions. Lots of folks have commented on the effects of the rulings (like this guest post at Sentencing Law and Policy).