Tags are keywords characterizing the references. Some tags are downloaded automatically with the references, but you can also add tags manually.
It is not recommended to keep the tags which are automatically downloaded. Each source has its own keyword system, so there will not be any logic in your tags. To deactivate the automatic downloading of tags, menu Edit -> Preferences -> General and uncheck "Automatically tag items with keywords and subject headings".
To assign a tag to several references at once, select them and drag them onto the tag you want to use, at the bottom of the left pane.
If some tags are particularly useful to you, you can assign them colours (by right-clicking on the tag and selecting Assign Color) and a little coloured square will help you to identify the references with these tags. There are nine different colours, so you can use this function for nine tags.