Actually, I do know what this means.

Remember how, with the older busses, you had to step up once to get on, and then negotiate another, relatively large, step?

The new non-step busses eliminate the second step. A big joy for oldsters and others of limited mobility.

Er, we have them in the UK. They can decend to meet the pavement level, it’s quite strange if you’re on the top deck and suddenly slide sideways.
We use “non-step” and somethingelse, which I can’t think of now, maybe low-rider or sumin…