Mostly yes. Is is the present-tense form of the verb wees (to be) used here.
In this sentence, it links the subject to a description:
A helpful point: Afrikaans uses is with all persons in the present tense:
So it is simpler than English, where you have am / is / are.
Because here klein is being used as a predicate adjective: it tells you what the cavity is like.
So:
If you put the adjective directly before the noun, you get a noun phrase instead:
Both are correct, but they do different jobs:
Not in the way you would in languages like German or French.
For ordinary learning purposes, Afrikaans nouns do not force you to choose different articles based on masculine, feminine, or neuter gender. You simply use:
So with holte, you do not need to memorize a gender category to build this sentence correctly.