Okay, so lets rewind a bit. Roundup is a brand name used by Monsanto for a type of herbicide called "glyphosphate" While it was under patent protection for 20 years, those patents all expired back in 2000 so now many companies market similar herbicides. These products are used by farmers to kill off smaller plants like grasses and bacteria that spread in the wild and would use nutrients in the soil that the farmer would rather be absorbed by their crops. It does this by blocking the creation of a few key proteins needed by all cells. This doesn't affect insects for the most part as animals don't make these proteins ourselves, we have to eat them ("essential amino acids"). But in plants, it kills them by denying them these required proteins.
Enter "Roundup Ready Soybeans" (and others, soybeans were the first though). They took genes from a few sources (a specific bacterium, a virus that infects cauliflowers, and a petunia flower) and combined to create a new method to synthesize those essential proteins, specifically one that glyphosphate doesn't block. So this means that glyphosphate will kill all the weeds, but leave the plants unharmed because they have a second way to make those proteins. The only thing added to the plan in the end is the same proteins it requires to not die (as do all cells). It should be noted that glyphosphate itself is still pretty nasty stuff, low levels of exposure are safe as humans don't make those proteins, but high doses (like straight from the can) do lots of other nasty things. So basically wash your produce and don't drink herbicide.
Totally separate is a group of GMO plants generally referred to as Bt plants, developed by a company that is now a part of Bayer, Bt plants start from a bacterium also called Bt, hence the name. Bt, Bacillus thuringiensis, produce a chemical called "delta endotoxin" when they go into their spore state, likely evolved to prevent them from being eaten by other microorganisms while they are inactive. Bt plants take the genes for that endotoxin and add it to the plants cells so they produce low levels of it continuously. The key bit is that delta endotoxin is a very selective attacker, it goes after a few proteins found in the stomachs of specific insects and paralyzes their digestion, eventually leading to death. Humans, and basically everything else, do not have these attack sites and so delta endotoxin is safe for consumption at any level unlike glyphosphate. It is also worth noting, when talking about poisons in plants, that the first commercial Bt crop was tobacco, well known and prized for containing the poison nicotine which does almost exactly the same thing in terms of being a natural insecticide. Caffeine is believed to have evolved for the same reason, though for a natural dose to be considered toxic you need to be the size of a bug.
Hope that clears things up!