A Knowledge Base Tag is a word or group of words that you can add to any article that can help you to link or relate articles together. You can use multiple tags in the same article. Tags can help you track your products articles, if you were to tag all articles relating to the same product then a search for the product tag word it would bring up all the articles with that specific tag. This is a good way to help your users self diagnose an issue they have with a particular product.
You can add tags within the Tags section within your Knowledge Base.
The other way is by using the Tags tab at the top of the Knowledge Base page and selecting the Add Knowledge Base Tag at the top right hand corner button and this will bring up a separate form to fill out.
You can add them when you create an article and typing in the Tags field this is a way you are able to add multiple Tags.
Give a name to your Tag that most appropriately represents what you require from the Tag. If you want to group all the articles about a product you support, then adding the product name as your Tag is an easy way to make the articles searchable.