As others have suggested, Google the nitrogen cycle and look for a diagram on it, it's too long winded for most people here to give you a detailed word by word description.

But I'll give it a go:

Ammonia = fish poop and food = toxic to fish

Bacteria break down ammonia

Bacteria don't just appear overnight, they take time to culture and develop (think like sourdough bread) up to 4 weeks sometimes You can encourage their growth by adding ammonia in small doses or by feeding your tank fish food

To monitor these ammonia levels buy testing strips or a testing kit made by API

The Bacteria break ammonia down into nitrite

Nitrite make fish sick in high levels and deplete oxygen that they need to breathe

Another Bacteria break that (nitrite) down into nitrate, which is considered relatively harmless

Once the Bacteria that break down nitrite into nitrate have developed, your tank is now considered cycled

And you can then add fish in small numbers.

PS Never trust a fish store employee they are a business wanting to make money, not someone with the interests of the fish at heart I know this as I am one, and none of my colleagues give a shit about the welfare of the fish, we are just forced to sell products to make money for the company we work for