Afrikaans is is the present-tense form of to be here, just like English is.
A very useful thing for learners is that Afrikaans verbs usually change much less than English verbs. With wees (to be), the present form is is used with all persons:
So in this sentence, is simply links the subject Die leiband and the description lank.
That is a very common learner question.
After is, the adjective is in its predicative form:
Before a noun, Afrikaans often uses a different attributive form:
So:
This is just one of those adjective patterns you get used to with exposure.
Afrikaans very often writes compound nouns as a single word. English sometimes uses two words where Afrikaans prefers one compound.
So leiband is the normal spelling, not lei band.
You would use 'n, the Afrikaans indefinite article:
A useful detail: 'n is written with a lowercase n and an apostrophe, even when the sentence starts. In careful writing, you would often capitalize the next word if it begins the sentence, for example:
In speech, 'n is usually unstressed.