What is the drum like thing in a cell tower?

Thanks for asking this question. It is an antenna. Point to point type. These antennas are highly directive and can haul wireless signals over several kilometers in a very narrow beam.

As antennas are enclosed in a protective gear it can be a bit difficult to know what kind of antenna it is.

To guess the kind of antenna the drum like shape is look at it from behind. If it is tapered towards the end then it is probably a horn. If it is round both front and back then its parabolic. The horn aperture is a lot smaller than the parabolic aperture. It looks more like a bulb than a drum.

Both horn and parabola are aperture antennas. Horn uses the principles of waveguide to provide and smooth impedance matching (in simple words transition for the signal in wire to the signal in free space aka air).

Where as a parabolic dish uses the principle of reflection to focus the signal and aggregate into a beam to transmit it long distance.