I assume that it means to heat the beer to 120 degrees. 10-20 seconds in the microwave should do the trick. It's just like making beer - they yeast need a warm environment to be comfortable. Bread yeast just needs a warmer climate than ale yeast. I find that 110-115 is usually the sweet spot. Just be sure, like brewing, to take the temperature and if you overshoot it, wait for it to come down or you could shock or kill the yeast.
When following bread recipes I usually heat the liquid first, then sprinkle the yeast over the warm liquid to let it rehydrate, and then sprinkle in a tiny portion of the dry goods (e.g., flour; exact measurements depend on the recipe). I let it sit in a warm spot for a bit to let it proof and get the yeast going. It's the same concept as a yeast starter. Scratch that, it IS a yeast starter. Then I continue with the recipe and let it do it's regular rise, which will be helped by the proofing. Healthier yeast makes for a more airy loaf.
Baking and brewing share a history for a reason. Although the part that amuses me is that, traditionally, the men did the baking and the women did the brewing. Bread was considered a man's job, and beer was considered a feminine responsibility