Are disc brakes weak? And what is "bedding in"? I've owned a couple of department store bicycles in the past with caliper brakes and I set the brakes up properly so when I squeezed the brakes the bike stopped pretty quickly.

I test-rode a bicycle from a bike shop a couple of days ago which had disc brakes, and it was just so slow to stop the bike, it seemed like the brakes were so weak. I checked the brakes on the spot by standing next to the bike and squeezing the front brake and pushing the bike forward, and there was no slippage, and same with the rear wheel. So I think the brakes were set up correctly, they just seemed to have no stopping power.

I asked the clerk about it and he said they would need to be "bedded in". I couldn't understand what he was talking about, but is it normal? And why can't I get a bike that's ready to go from when I buy it. Do I just buy a bike with weak brakes and use it for a while and after a few days the brakes will work better after it grinds a groove in or something like he said? Or were the brakes not set up correctly?
Bedding-in is a common process with new disc brake pads. Disc brakes were developed much earlier for motorcycles and cars, from which the technology and the terminology has been transferred to MTBs: see https://ebcbrakes.com/articles/motorcycle-bed-in/ and http://www.centricparts.com/files/White%20Paper%20Revisions%204-2012/Centric%20White%20Paper%20C3-2012-Bed-In%20Stock.pdf

Bedding-in is a process in which both the pad surface microstructure but also the surface composition of the rotor is affected. During the run-in a very thin layer of material is transferred from the pads to the rotor. This surface deposit increases the brake power significantly. 

However, even when the brake pads are not yet bedded-in, a disc brake should have more stopping power than a rim brake. So I wonder whether the surface of the rotor or possibly the pads have inadvertently been poisened by a lubricant. Ask the seller to clean the rotor with alcohol.