A will contest is a type of lawsuit that is brought to challenge the validity of a Last Will and Testament and can be filed at one of two points during the probate process.
Before the will is admitted to probate. A pre-probate legal action is called a caveat. In essence filing a caveat puts the public and, more importantly, the probate clerk, on notice that a decedent's Last Will and Testament cannot be admitted to probate until notice of the filing of the will and the Petition for Probate Administration with the probate court is given to the person who has filed the caveat, called the caveator.
If a caveat is filed, then the will cannot be admitted to probate until the caveator has had the opportunity to respond to the Petition for Probate Administration.
Also note that in a handful of states, including Alaska, Arkansas, Delaware, North Dakota and Ohio, a person who makes a will or trust can "pre-validate" it while they're still alive. This, in turn, will prevent the will or trust from being contested after the person dies.
After the will is admitted to probate. Once a will has been admitted to probate, the Personal Representative/Executor has the legal authority and obligation to settle the decedent's estate, and so the Personal Representative/Executor will start the steps necessary to complete the probate process. As part of this process, the Personal Representative/Executor is required to notify all of the beneficiaries named in the will of its existence and the probate proceeding through a legal notice called a Notice of Administration in some states. Depending on state probate laws, this legal notice to a beneficiary will limit the time period that the beneficiary has to challenge the validity of the decedent's will, usually only 30-90 days.
The Personal Representative/Executor may also elect to send the legal notice to anyone not named in the will who could file a will contest, including heirs at law or beneficiaries or fiduciaries named in an older will who have been cut out of the new will. This is good practice particularly if the Personal Representative/Executor believes that a will contest could be filed, otherwise the time frame within which a disinherited beneficiary or disengaged fiduciary can file a will contest may be unlimited depending on applicable state law.