Well I agree it's not that clear, but there is this passage there:
Clicking on a tag activates it, and only notes having this tag will be displayed. Using Ctrl+Click or Shift+Click, you can expand the tag selection to additional tags. Keep in mind that with Ctrl+Click, a tag intersection is used when multiple tags are selected (i.e. notes having all of the selected tags will be included), whereas with Shift+Click, a tag union is used (i.e. notes having any of the tags will be included).
There is also a possibility to exclude (subtract) tags from the set via Alt-clicking on them. The excluded tags will show as strikeout with a bluish background. Only the notes which have been selected via tag union or tag intersection tags, and which additionally don’t have any of the excluded tags, will be shown in the list.
The simplest way to clear all tag selection is to click on "All" pseudo-tag up above.