'n is the Afrikaans indefinite article, meaning a or an.
So:
The apostrophe is just part of how the word is written. It is a very short, weak word in speech, usually pronounced like a quick uh sound.
A useful thing to know: even at the start of a sentence, 'n usually stays lowercase. For example:
Roughly:
A rough pronunciation of the whole sentence is:
That is only an approximation, especially for tuin, because the Afrikaans ui sound is difficult for English speakers and does not have a perfect English equivalent.