What is the definite article (al-)?

Attach الْ to the front of an indefinite noun and it becomes definite — and the tanwin falls away: كِتَابٌ (“a book”) becomes الْكِتَابُ (“the book”). Before sun letters the lam assimilates in speech — الشَّمْس is pronounced ash-shams — though the spelling keeps الْ. Together with idafa, this is one of the two main ways Arabic makes a noun specific.

Two things are easy to confuse here.

First, الْ is not always for definiteness. It also arrives as part of a name (الْحَسَن), or to make a class statement — الْإِنْسَانُ ضَعِيفٌ, “man is weak”, meaning all of humankind.

Second, sun and moon letters change the sound, not the grammar. اَلشَّمْس and اَلْقَمَر are equally definite; only the ل of the first is assimilated in pronunciation.

Does ال always make a word definite?

No, and that is the trap. It also arrives as part of a name and stays there, and it is used to make a statement about a whole class rather than about one member of it. Definiteness is its commonest job, not its only one.