Okay kiddo, so in the Ahmadiyya Muslim community, the idea of prophethood is very important. A prophet is someone who is chosen by Allah (God) to receive a message and bring it to the people.
The founder of the Ahmadiyya community, Mirza Ghulam Ahmad, claimed to be a prophet sent by Allah. He said that he was not a new prophet, but a follower of the Prophet Muhammad (peace be upon him).
Now, some other Muslims do not believe that prophets can continue to come after Prophet Muhammad. But Ahmadis believe that Allah can choose whomever He wants to deliver His message, even if they are not from the same community or religion.
So when Ahmadis say that Mirza Ghulam Ahmad was a prophet, they mean that he was chosen by Allah to spread a message of peace, love, and unity among people of all religions.