This is the 50th most frequent Finnish word.
yksi
"Yksi" means "one" in English.
In this sentence, 'yksi' means the numeral 'one' indicating a single item.
Täällä on yksi kirja pöydällä.
There is one book on the table.
Here, 'yksi' is used to signify 'one specific', emphasizing the particular thing being referred to.
Yksi asia huolestuttaa minua.
One thing worries me.
In this case, 'yksi' precedes 'asian' (thing) and is used to specify 'one' in a countable manner.