A silo is something which is isolated from something else.
In organizations, silos happen when teams focus solely on their respective departments and team-specific goals. Very often, the goals of one team may clash with the objectives of another. This can create siloed environments resistant to information-sharing.
As a team leader, you should strive to share information and keep open lines of communication with neighbouring teams and team leaders. The ideal is for the people around you to know what your team is doing and why, and for your team to know the same of other teams.
Maintaining an easily accessible team playbook is a great way of facilitating this responsibility.