Hashtags are keywords to help organize posts on DIY. To 'tag' your posts, just add any word or group of words (without spaces) preceded by the pound sign (#) in the comments. On DIY, this will cause the hashtag to turn into searchable link. Click on a hashtag to see all the posts that mention the subject in real time.
For example, if you posted a video explaining how to bake a raspberry pie (yum!), you might tag your project with #pie, #recipe, #how-to, and #baking.