No, not guarding.
Weeds are plants that grow wild and usually are broad leafed. That is, they have broad leaves rather than being narrow like grass. Weeds are not wanted in fields where crops are being grown, in flower beds, in vegetable gardens, etc. They grow over the crops, compete for nutrients from the soil, water and sunlight which can cause the crops to not grow well. Geese can be trained to eat those weeds and leave the crops alone. And throughout history that have been used by commercial growers to weed (eat the invasive, wild plants) in their fields. Today, usually people think of weeding as how much grass and vegetation the geese like to eat though because weeder geese have mostly been replaced by herbicides (chemicals that kill the weeds) and machines in farming. The column on that chart shows how well each breed can be trained to do that and/or how likely they are to forage for weeds to eat.
Does that help?
