The whole family was at camp last week, living in tents, sleeping on cots, eating in the mess hall. Every camp has them, squirrels and chipmunks that survive on the crumbs of campers' treats (or sometimes the whole banana). We were warned - no food in the tents except in metal boxes.
The boys had the tent next door to ours. I came back from dinner one night to find a very happy squirrel just making off with a chip container from the kids tent. At which point I remembered the dried fruit I'd left in my pack after the morning hike. Whew...it was still there. The rodents had been attracted to the far more tasty snack leavings next door. The boys tent is serving as (a chemist would say) a protecting group.
Chemical protecting groups work similarly. Say you have two sites on a molecule that can react with a reagent, but you only want one to undergo the reaction. If you can put a protecting group on the site you want left unmolested, like a cover, you can run the reaction, change the other site and then take off the protecting group. (See the scheme for an example.)
It works wonderfully for many reactions, and is keeping my pack safe from marauders.