Back tire to fell off while driving

My rear tire on my passenger side had oil leak and a smell. What cause a alarm is that there was smoke coming from the tire. I had a friend look at the tire and the I was told that I needed brake drum, brake shoes and a wheel bearing, but before knowing all of that before the tire was taken off my friend was able to move the tire in and out and he could not understand why it was doing that. When he remove the lug nuts and took off the drum he was confused to what was holding my tire in place, because he said that my tire was just there and nothing was holding it. Now I was told I may need a back braking plate, or axle seal retainer. I a a bit confused because I do not know what holds a tire in place and what a Suzuki has to hold a tire in place.