The idea is that you will choose a product that someone may have interest in. By getting this product in front of your audience you can make a commission if someone makes a purchase.
There are different strategies I use to accomplish that.
For one, I built a website by starting a blog.
Let's say your a woman and your into beauty and makeup products. You can write content about a brand and their products. That content gets ranked on Google. When people search on Google about that product or brand, your website will show up, they will see it and click it. On your website, you put a link to the product your reviewing or an alternative that can be brought on Amazon and a visitor goes to check it out.
When that person goes to Amazon and buys ANYTHING within 24 hours, you will earn a commission. Let's say that she buys beauty and makeup products along with pet food for the dog or other things she might need.
You'll get a commission on the beauty products along with anything else purchased. Let's say that 10 visitors repeat the same and you have made $300 in a day. Do you start seeing the potential?
There are more than 4 BILLION people on the Internet (and counting!) so the potential is huge. You just need to get a tiny fraction of that huge mass and you are already making a nice income.
Another strategy I use is to pay for advertisements to promote products on various platforms. This does not involve blogging or publishing content as the main goal is to get someone to take action, either by purchasing a product I am promoting or building a list to follow up with potential buyers.
These strategies of promoting products and earning a commission is called affiliate marketing.