It doesn't. The only way an ice sheet gains mass under any normal circumstance is new snow falls on top. As more and more snow accumulates over time, the ice sheet grows thicker.

What offsets that is that ice flows under pressure, so the entire sheet has a tendency to head for the ocean. Things are a bit complicated in Antarctica since some of the landscape where the sheet is grounded is below sea level. There are concerns that the seawater is getting warm enough to cause melting there, increasing the flow rate.

If, in fact, ice sheet mass is increasing, that's great in terms of sea level rise. There's no other way to spin it.

You have a layer of ice covered with a layer of snow. Each year, that snow melts a little and then re-freezes on top of the ice. Then, more snow falls on top of that snow. This happens year over year for a long time. Each layer of snow puts more and more pressure on the previous layers of snow and ice, compacting them. Eventually, these layers of snow become so compacted that they form new layers of ice and fuse into the existing layers of ice. Does that make sense?

I assume it means snow melts, turns into water, then freezes again turning into ice thereby increasing the mass of the ice sheet.

That really isn't how mass works.