Carder-bees are to be found in long meadow grass, where they gather moss and form a snuggy ball shaped nest. When disturbed some bees will fly out and make an aerial display, while those in the nest perform a communal buzzzz. They're telling you to back off and can sting, but only if you persist in annoying them.
Most people come upon Carder-bee nests when trying to clear an overgrown garden and especially when cutting long meadow grass.
Carder-bee nests can be moved but it is far better to leave them in place, if possible. You can finish the garden work at the end of the summer when the bees have finished. These bees are mild tempered and are usually a pleasure to have in a garden. They will not return to the same spot the following year.