Al and Definiteness (ال و الوضوح-)

The definite article (al-) was mentioned briefly when talking about Sun and Moon letters. It acts like the word "the" to indicate that a word is definite.
Compare the samples below:

InDefinite

Definite

There are other signifiers of definiteness, such as possessive pronouns, which are attached to the end of the word and imply ownership:

InDefinite

Definite

Likewise, Proper Nouns express definiteness.

InDefinite

Definite

Demonstrative pronouns, such as "this" "that" "these" and "those," of
which you know a couple, هذا and هذه , are also indicators of definiteness.

InDefinite

Definite

There is an Arabic construction called an Idaafa Phrase that also can be used to reflect possession or specificity, and subsequently Definiteness. Construction of Idaafas will be explained later, but here are some examples so you are familiar with what they look like.

InDefinite

Definite

Overall, Definiteness is this sense of specificity. When the object is definite, you know that it is not some random object that goes by the name of the object category, but a specific object in that category. For example "chair" could mean any possible chair in the world, while "the chair" is one that is being talked about or looked at, "his chair" is the specific one he owns, "Maahaa's chair" is the chair that Maha owns and "The Iron Throne" is the one and only chair by that title. This should give you a sense what Definiteness means which will help when understanding agreement and equational sentences.