I have a project which uses yolov5. I modified the yolov5 and implemented it into my own project (I don't just use the modified version of yolov5 as project. It is just a part of my project.). I read the GPL3-0 license but it contains a lot of terms so that I couldn't fully understand it. Can i sell this project as a product and earn money, is this legal?
Yes, the GPL v3 (like all free/libre/open source software licenses) allows you to sell derivative works (or unmodified versions). However, it is a copyleft license, so you must release your derivative work (and possible your entire product) as FLOSS under the same license. You should talk to a lawyer about this to determine what does and what doesn't need to be GPL'ed, unless you want to make your entire codebase FLOSS.
The rules are simple: When you ship the software, you must ship it together with the GPL license. You are allowed to do the following:
Sell the software together with the GPL license, for any amount of money that someone is willing to pay you.
Charge reasonable cost for providing the source code if someone asks you for it.
You are not allowed to charge for the GPL license itself. You must provide that at no cost.
As "Someone" commented elsewhere, if you sell software to businesses, they would legally be allowed to hand the software for free to a competitor, but as a business, you don't give your competitors anything of value for free. So you can likely sell to competitors as well.