What does the "@" symbol mean in Bazel?

This is to reference a remote repository.

From the doc, depending on other Bazel projects

local_repository(
    name = "coworkers_project",
    path = "/path/to/coworkers-project",
)

If your coworker has a target //foo:bar, your project can refer to it as @coworkers_project//foo:bar.

See also the design doc of remote repository and bind example in workspace rules.