In Finnish, jk is the abbreviation for the word jokin, meaning something.

Who cares?

Well... Finnish is an inflected language, in that nouns (and adjectives) have many cases according to their use in a sentence. Genitive denotes possession; Partitive denotes an unknown quantity and so on.

In a dictionary, an abbreviation of jokin will appear in the correct case for a given verb.

For example, pitää (to like something) takes the Elative case, which normally denotes motion out of something, but in this case is just the object of your affection.

Se menee talosta - He goes out of the house.
Se pitää kahvista - He likes coffee.

So, in a dictionary, pitää would appear as "pitää (jstk)" to show that pitää jostakin is "to like something".

There follows a list of cases with abbreviations, and full version:
It is possible to learn Finnish for many years without this little piece of knowledge, but it gets much easier once you know it.