There is a curvy part of pipe called a p-trap underneath the toilet. All drainage plumbing in your house house these p-traps. They are under your sinks, showers and bath tubs. They create a valve out of water which prevents sewer gasses from coming through your plumbing.

The toilet is essentially a very large P-trap. When you flush the toilet it begins to fill the bowl with water. The water continues to rise until the volume of the water in the p-trap can no longer support the weight. When this happens the toilet will flush. The toilet has multiple specially designed pathways for the water to travel which help maximize the flushing action.

After the toilet flushes it automatically begins refilling the tank the the bowl simultaneously.
A floating valve inside the toilet tank shuts the water off once its refilled.

Onetime I went in vacation during the summer and turned the ac down. A few of the p-traps in our house had dried up and the kitchen wreaked pf sewer gasses. When leaving your house unattended for extended periods of time, it may be worthwhile to ask a neighbor to come by and run your water in the sinks and showers. This will ensure the water valve remains in place and well prevent sewer gasses from getting in which is gross.