Your toaster does use a reasonable amount power but for a short period of time so realistically it's negligible unless you're using your toaster to try and heat your house.

The wiring in your walls is probably rated for 15 amps, depending on when your house was built the gauge and length of that wire run will vary. Along that wire run there are multiple plug sockets - again, number will vary with age of home.

Let's say your toaster is plugged into the last socket on that wire that runs back to the breaker box in your house - so it's using the full length of the wire. You turn it on and the heating coil wants to pull 15 Amps (1800 watts on a 120v) to heat up. So on one end of the long wire, you have 1800 watts being pulled, and the other end your breaker box - and that's the full capacity of the wire.

So what happens when something plugged into one of the sockets along the run is needing to draw power? Weird stuff! The voltage and power availability is affected by what's called "sag" and it's having to fight against the draw of the heating coil. If you wanted to make it even more funky, plug a motor like a blender into the same circuit - as many people do in the kitchen.

Basically, you're going from the illusion of how electricity works ("you plug it in and it just works") to the reality of how electricity works ("holy crap connecting a bunch of high power devices with thin bits of wire causes problems").