This is the 4th most frequent Finnish word.
mukaan
"According to," "along," or "with."
In this sentence, 'mukaan' is used to express participation or involvement in an activity or group.
Hän haluaa liittyä mukaan projektiin.
She wants to join in the project.
Here, 'mukaan' and its possessive suffix '-si' indicate bringing something along or accompanying.
Ota nämä kirjat mukaasi matkalle.
Take these books with you on the trip.
In this instance, 'mukaan' illustrates conformity or correspondence to a standard or set of rules.