Tags (as used on this website) are essentially the same thing as hashtags. The only difference is that all tags contain at least one underscore, and tags do not begin with a #.
When composing a message, press the Tag button and type in a tag. If you type spaces, they will be converted to underscores. If you only type a single word, it will put an underscore on the end of it. The underscores let searches recognize them as unique. Without them, it would often include things without the specific tag.
Selecting text and pressing the Tag button also works.
If you select multiple lines, it will make a new tag out of each line. If you add commas or semi-colons, it will treat each space between them as a new tag.