Learn Russian: Трамвай едет медленно.

In Russian, you must choose a different verb depending on whether the motion is on foot or by transport. Идёт means going on foot, but since a трамвай is a vehicle, we must use едет (goes by transport).

Does едет mean the tram is going slowly right now, or that it generally goes slowly?

It means the tram is going slowly right now in one specific direction. Едет is a one-way motion verb. If you wanted to say that this tram always goes slowly in general, you would use a different verb: ездит.

Why does the word медленно end in -о?

The -о ending tells us this is an adverb (a word describing an action). It answers the question "how?" — how does the tram go? Медленно (slowly). If you wanted to describe the tram itself as a "slow tram," you would use the masculine adjective form: медленный трамвай.

What gender is the word трамвай?

It is masculine. While most masculine Russian words end in a regular consonant, words ending in the letter -й (short i) are also masculine.