It really depends on the production, and what the needs are of the company/person hiring you.
It could be fairly similar to a more traditional play, just with fewer cues. It could be presetting props, and then being the person who communicates with the HM throughout the evening. Sometimes it's just making sure that all of the physical needs of the production are being met- are there enough music stands, do we have pencils, where's the bathroom/water fountain, etc.
Sometimes it's just being there, documenting curtain up/curtain down times, making sure that the performers show up, and that the building doesn't burn down.
But honestly, I think it's a great question to ask during an interview for a position like this. "What are your expectations for the stage manager?"