The probate process has two purposes: to make sure the will is valid under the law of the state where it is being probated, and to wrap up the estate's affairs so that the beneficiaries in the will can receive the assets the will gives them. While the probate estate is open, the probate court hears issues relating to whether the will is valid and how the will's instructions are to be carried out. The probate court only closes an estate if no more issues about the will's validity are present and the estate has been dissolved.
A will contest or a will challenge contests whether a will is valid under state law. Common reasons for a will contest include that the person who made the will didn't have the mental or legal capacity to make a will, that the person who made the will was forced, coerced or tricked into leaving his property the way he did, or that the will itself is partly or totally a forgery. If the will is found not to be valid, the property will be distributed according to the state's rules, as if no will ever existed. All these events must take place while the probate estate is opened and must be resolved by the probate court.
Many states have a statute of limitations that shortens the amount of time you have to file a will contest after the will enters probate. In some states, this period is only a few weeks. Once the limitations period has passed, you cannot contest a will even if it is still in probate. Part of the reason for state statutes of limitations on will contests is to ensure that there are no issues about the will's validity when the estate is finally distributed to the beneficiaries listed in the will.
Even if a state accepted a will contest after probate had ended, there are several reasons why filing a will contest at this time would make the contest much more difficult to win than if it was filed during probate. First, the longer you wait to file a will contest, the less likely you will be able to find evidence that the will is invalid. Witnesses, documents and memories of the will's creation may easily be lost with time. Second, many wills contain a "no-contest" clause that requires beneficiaries who contest the will to give up their share of the estate if they lose. This share is easier to give up if it has not been distributed yet.