Resolution is measured in megapixels (MP): 2MP ≈ Full HD, 4MP gives more detail, 8MP ≈ 4K. The higher the resolution, the better details such as faces or number plates show up — especially when you zoom into a recording.
Higher resolution also means bigger files: more disk space and more network load. That's why resolution is chosen to match what each area actually needs.
Example
A 4K camera above the cash desk so banknotes are clear; 2MP is enough for a corridor.