Like, how come companies pays a lot of money to buy the source code and now this is open to everyone ?
If let’s say im building a game using the source code and it is ready in about 3 years, will i be able to sell it and pay only 5 % to epic ?
I mean, is it that good ? 
You can pay $19 for one month and then cancel so that you can download the editor. After that you can develop a project using it and once you are ready to sell it you will then have to pay 5% royalty.
If you continue paying subscription you will have access to new builds that will have bug fixes and new features, you will also get access to the marketplace and you will also get access to the source code.
And yes, the source code was something you did not get with UDK, so having it with UE4 is a big deal, it allows you the ability to do a great deal with the engine that you couldn’t do with UDK. Most people won’t need it, but it will allow people to build tools that integrate with the engine or even add support for hardware and other things.
Thanks ! 