This is the 2198th most frequent French word.
dormir
To sleep.
Here, 'dormir' is used in its infinitive form to express the action of sleeping intended in the near future.
Je vais dormir tôt ce soir.
I will sleep early tonight.
The word 'dort' is the conjugated present tense of 'dormir' for the subject 'il', describing ongoing sleep.
Il dort profondément après une longue journée.
He is sleeping deeply after a long day.
Here, 'dormi' is the past participle of 'dormir', utilized to indicate the completed action of sleeping in the past.