This is the 4th most frequent Slovak word.
je
"Je" in Slovak means "is" in English.
In this sentence, 'je' is used as the present tense of the verb 'byť' (to be), linking the subject with the adjective.
Dieťa je šťastné.
The child is happy.
Here, 'je' serves the same function but is linking the demonstrative pronoun with the noun.
Toto je kniha.
This is a book.
In this context, 'je' is used as the present tense of the verb 'jesť' (to eat), meaning it is an action performed by the subject.