That's really about as simple as I can make it, but I can try I guess.
The EZ-V 3in1 has two things you can use to run GBA games - RAM and NOR flash. The NOR flash is the type of stuff used in old-school GBA flash carts. It takes a long time to write to, but the data is saved on the 3in1 even after poweroff. In essence, the 3in1 becomes recognized as the game you've flashed to it. There is 32 MB of this NOR flash, and the largest size any GBA game can be is 32 MB, so it works out.
There's also 16 MB of RAM on the device. This was originally added so that the Opera web browser could be used with it, but people later found out how to load GBA games into it and launch them. This is much quicker to load, since it's faster to write to than NOR, but the data is lost after you power off the device (save files aren't lost though, so don't worry about that). Since there's only 16 MB of this RAM, you'll need to use the NOR for games that are 32 MB in size.