Click on the add button under Tags and choose the tags to add. To remove a tag, click on the x next to tag.
Custom tags can be created and made available in the Settings tab or in the pop-up to add a new tag. Project owners and those with permissions to configure project settings can create tags.
Users with view and edit permissions in bugs can add and remove tags. Depending on the settings of the user's project permissions, they may be able to view all the project bugs or just a subset of them. Users can also be specifically added as a watcher to view the specific bug.