When the giantess Grid hears that Thor's planning to venture into Geirrod's territory without any weapons or armor, she thinks it's a pretty stupid move. So she gives Thor some gifts—a girdle (or belt), staff, and iron glove—that become very useful to him later on in the story. In fact, it's fair to say that Thor wouldn't survive without these gifts.
Although our super-macho hero might not want to admit it, these gifts represent Thor's dependence on the wisdom and kindness of another for his survival. But individually, each of these gifts has a unique symbolism and a long history in mythology and literature.
Let's break it down
You might be wondering what a girdle actually is. Well, that depends on when you're wearing it. Today (and starting in about the 1920s) a girdle is an elastic waist-cincher designed to hold women's stomachs in (the things women do for fashion, right?).
But for most of history, a girdle was just a belt. When a woman wore a girdle, it usually symbolized her sexuality—in particular, her virginity. But because a man used a girdle to hold his sword, it symbolized his power and protection. How about some examples?
Thor actually already has a girdle. When he wears it, it doubles his strength, basically making him undefeatable. But he left his personal girdle home on this trip. Lucky for him, he found a spare.
Who do you picture when you think of a staff? Probably a wizened, elderly man with a long, white beard, right? Wizards—guys like Gandalf and Dumbledore, for example—often carry staffs, which, in addition to possessing magical powers of their own, symbolize authority and wisdom.
In the myth of Thor and the Jotun Geirrod, the staff might symbolize Grid's wisdom, which Thor uses as he ventures into the giant's territory. It might also symbolize Thor's journey; since medieval pilgrims often carried staffs to use as walking sticks, the staff is also a symbol of travel.
In the Middle Ages, everyone—not just fancy ladies—wore gloves. Gloves came in pretty handy when you were doing hard physical labor. In battle, armored gloves could protect your hands. And since without your hands, you can't do much, hands—and by extension, the gloves people wore on them—came to symbolize power.
A lord might express his permission for a vassal to found a town or possess a piece of property by giving him a glove. Torqued off at someone? You might express your desire to do battle with them by throwing a glove on the floor in open court. You've "thrown down the gauntlet," which is just a fancy word for glove. If the offender picks it up, then the fight is on.
So in the myth of Thor and the Jotun Geirrod, Grid's glove might symbolize the transfer of her "power" (in the form of knowledge of Geirrod's tricky ways) to Thor. It also tells us that it's "game on" for Thor and the giants.