Regarding these 13 principles I prefaced some remarks on the parent page here
The Rambam’s sixth principle is regarding prophecy. I paraphrase, approximately, adding how I understand his words more or less: There have been people who merited to reach a high level of ability, and perfected themselves to a high level, to the point where their soul is able to attach itself to the Creator, and thus receive a higher manifestation of spirituality from the Creator.
These people are prophets, and this is prophecy in a nutshell. The full explanation of this is very long, and the Rambam says that it is not his purpose to cover the matter entirely, only to mention that which is necessary for these matters (to explain the mandatory creed).
The Torah itself testifies about the prophecy of many prophets.
