This is the 22nd most frequent Czech word.
jak
"Jak" means "how" in English.
In this sentence, 'jak' is used as a question word asking 'how'.
Jak se máš?
How are you?
Here, 'jak' is used to mean 'as' or 'in the way which', connecting the manner of doing to the statement.
Dělala to přesně jak jsem řekl.
She did it exactly as I said.
In this context, 'jak' introduces a dependent clause indicating the manner of describing.