What is the meaning of -다?

a plain style declarative suffix

declarative mood sentence-final verbal suffix

In the plain style, the sentence-final declarative mood suffix.

Used to form the dictionary citation form of verbs and adjectives; note that Korean verbs and adjectives are never lemmatized at the infinitive.

Contraction of 다가 (-daga, emphatic marker).

Alternative form of 다가 (-daga, and then; but soon, conveys one action halting and another beginning).

Source: wiktionary.org