Good question :) Well, it is something you could have looked up on Wikipedia, it's honestly a good resource for this. But sure...
Rendering happens when you set up an image of some sort and then let the PC make a real picture out of it. So, for example: you have an environment in which you place some objects and also tell it where the main light comes from (on Earth that's normally the sun or some artificial lights). Well, the computer would then process that information and for example starts calculating which objects would cast shadows, and what effect those might have on other objects.
It'll continue to do so until you get an end result.
SO what do you need to do this for yourself? Well... a so called render engine. Of course the engine itself won't be enough, because how are you going to place those virtual objects? There are a few programs I can recommend...
A very good way to get started with this is
Daz3D. In specific you should grab Daz Studio and if you feel brave Hexagon too. Both programs are free to use and they are in my opinion the perfect way to get started.
Warning! Although everything I said is true you need to keep in mind that Daz Studio is basically a so called
freemium program. Yes, it's free of use and if you know your way around you can do some amazing things. Especially with all the free 3d models that are out there. But... to get the best results you're probably eventually want to buy some starter packs.
The reason I still suggest this stuff is because it makes it really easy to get started. And once again: it may be freemium, do not underestimate all the things you can do with it. This thing is
very sophisticated. You can even build your own shaders if you want to (not to be confused for the shaders in Minecraft)...
Another option is
Blender. This is a completely free (open source) program but... although it has a lot of potential it's not very user friendly. Expect a steep learning curve.
My suggestion is to grab all of the above and see where it gets you.
PS... if you plan on rendering (or building) sceneries (outdoor environments) I can highly recommend
Bryce too. This is commercial software, limited & dated at that, but it makes rendering landscapes a heckofalot easier.
Hope this can help.