1 Samuel 10:10 “And the Spirit of Elohim came upon him, and he prophesied in their midst.”
1 Samuel 11:6 And the Spirit of Elohim came upon Sha’ul mightily as he heard these words

The word prophecy in Hebrew is NABA H5012 – nāḇā’, and at its core it means to “be under the influence of a spirit“. 1 Samuel 10 and 11 confirms this, in that the Spirit (Ruach) of Elohim came upon him (even mightily). What happens is that the Spirit (or spirit if bad) can be so overwhelming, that the person starts speaking, preaching, warning or sing abundantly with emotion.

Back in Numbers 11:25-29 when Moses gathered seventy men to be the elders of Israel, ” and it came to pass, that, when the spirit rested upon them, they prophesied, and did not cease. .. And Moses said unto him, Enviest thou for my sake? would Elohim that all the YHWH’S people were prophets, and that the YHWH would put his spirit upon them!” So according to Moses, YHWH wants His Spirit on all His people and all can be prophets. Prophesying is not just for some, and it is not just about making future predictions. It is whatever YHWH wants to communicate to you or the people around you, and the Spirit of YHWH is powerful in that it can include giving signs, reveal revelations and give dreams.

Off course, if the wrong spirit comes upon a person, then he or she is considered a false prophet. Deuteronomy 13 (click here to read more) is a whole chapter dedicated on how to identify a false prophet. If they do give signs, give revelations or share their dreams, and any of it contradicts the Torah in any way – they are considered a false prophet. Also, if they make predictions and it does not come true, they are a false prophet. But YHWH says in Deut 18:20-22 that we should not be afraid of them, if YHWH will deal with them at the end of the day.