Dependabot is GitHub's tool for keeping a project's dependencies up to date. It checks the packages a project uses and opens pull requests when newer versions are available. It can also raise alerts and propose updates when a dependency is affected by a known vulnerability, which is often called a security update.
Automated update requests make it easier to keep libraries current without regular manual checking. Because each update arrives as a pull request, tests can run against it and a person can review the change before it is merged.
Dependabot does not make a project secure on its own. It can only propose updates for the dependencies and ecosystems it supports, and it can only fix a vulnerability if a fixed version exists. An update can also introduce incompatibilities or regressions, so merging one without testing is a risk. A pull request being open means the fix has not yet been applied.
To use it well, review and test the proposed changes, act on security updates promptly, and group or schedule routine updates so they do not overwhelm the team. Combine it with other practices such as reviewing new dependencies, and check that merged updates were actually deployed.