This is the 51st most frequent French word.
donc
The French word "donc" means "so," "therefore," or "thus."
In this sentence, 'donc' is used as a conjunction meaning 'so', establishing a cause-effect relationship between being tired and deciding to rest.
Je suis fatigué, donc je vais me reposer.
I am tired, so I am going to rest.
Here, 'donc' connects the observation that it is late to the suggestion of leaving, again indicating a reason-consequence relation.
Il est assez tard, donc nous devrions peut-être partir.
It is quite late, so maybe we should leave.
In this usage, 'donc' serves as a transitional adverb equivalent to 'therefore', marking a formal conclusion drawn from the first clause.