When a post or comment includes a hashtag, the hashtag shows in the Trending Tags section of your dashboard. You can click the hashtag to open the search page for that hashtag and see all posts and items related to that tag. You can also search for hashtags using the main spyglass search in the upper right of the page.
The Trending Tags section is limited to a maximum of 10 hashtags, and shows results from the past two weeks (14 days).