A Silo is essentially a fixed inventory for tamed Black Ants to use for their own assigned roles (see their page for more information). It must have the things in it initially as dictated on the Black Ants page in order for those ants to be able to autonomously farm.
Silos have a custom GUI when opened. To construct one, you require one set of Silo Supports, one Silo Tank and a Silo Roof. Put them down one on top of the other in that order and the Silo will be ready to use.
To assign ants to a Silo, see Ant Medallions.