Not all languages have articles. English and Afrikaans are the same here. 'n Boek is a book, and die boek is the book. A ('n) is the indefinite article and THE (die) is the definite article.

'n and DIE: What is the difference?

Example:
"Hy is dors, gee vir hom 'n glas water asseblief."

VERSUS

"Hy is siek, gee vir hom die glas medisyne asseblief."

Here, 'n glas water means any glass of water. He is thirsty and he needs water.  When saying 'n glas water, you might have to find a glass (any glass) and pour water in it.

Then, die glas medisyne would mean that you have prepared a specific glass of medicine, and you want to get this glass of medicine that you have prepared based on his illness.

Note:

'n -- Always in lower case. If you start a sentence with 'n, the next word will be capitalized. e.g. 'n Man met 'n masker is in die bank.