What is the role of the petitioner in probate court?

A "Petitioner" is a person who brings a petition. They are kind of like a plaintiff in a civil suit. They are asking the court to do something, that is make an order effecting their, and potentially other persons, rights and property.

A "Respondent" is the person who (yup, you guessed it) responds to the petition. While similar to a defendant, the respondent is not necessarily adverse to the petitioner.

We use petitioner and respondent in Probate Court as the same person may be either a petitioner or respondent at different times in the proceeding.

For example, in California, a petitioner on a Petition for Probate is asking the court to appoint someone to manage the decedent's estate, generally called a personal representative and more particularly called an executor (will) or administrator (no will.) They might also ask the court to "admit" (think validate) the decedent's will to probate. The Petitioner normally is, but does not need to be, the person who will serve as the representative.

Now, lets say that a representative is already appointed by the probate court, and wants to sell a parcel of estate property. But one of the heirs objects.

The objecting brings a petition, called something like "Objection to Sale of Estate Property." The heir now becomes the petitioner and the representative, who needs to answer the petition, is the respondent.

Although this general answer is provided by an attorney, it should not be taken as legal advice regarding your particular situation and no attorney-client relationship is established. For help with your particular legal situation, please consult with an attorney.