Usually, it means teams don't share within the organization. Each team basically toss releases over the wall to whoever is next. For example, my team wrote a feature, toss the entire thing over to QA team for testing, then toss the thing over to DevOp team for deployment, then toss it over to SRE team for monitoring and operations.

A silo refers to a team or org that's solely responsible for some aspect of the product, with other people being unwilling, unwelcome, or unable to participate.