Learn French: Elle revient en bus. - She is coming back by bus.

Revient is the form of revenir (to come back) that goes with elle. In French, adding the prefix re- to a verb usually means doing the action again. Because it is built from the verb venir, it follows the exact same pattern: since she comes is elle vient, she comes back is elle revient.

Why do we say en bus here instead of translating 'by bus' literally?

In French, you generally use the short word en for modes of transportation that you can physically sit or stand inside. This is why you say en bus (by bus), en train (by train), or en voiture (by car).

Does revient mean 'comes back' or 'is coming back'?

It can mean both! French does not have a separate tense for things happening right now like English does with 'is doing'. The French present tense covers both general routines and current actions. Elle revient translates to 'she is coming back' here, but it could also mean 'she comes back' depending on the situation.