A multiband antenna is an antenna designed to operate in multiple bands of frequencies. Multiband antennas use a design in which one part of the antenna is active for one band, while another part is active for a different band.
Techopedia explains Multiband Antenna
Multiband antennas may have lower-than-average gains or be physically large in comparision to single-band antennas in order to accomodate the multiple bands.
The wires at the center of the feed of simple multiband antennas are separated vertically by a small amount, and the ends are separated by a few inches. These simple dimensions make it possible to cut antenna lengths for given frequencies and eliminate the need for pruning.