Hover over a topic and an ignore icon will appear at the end of the topic's row; clicking it will hide the topic, and it will be hidden on subsequent visits to the forum it's in, and on the Recent Unread Topics page.
On the Recent Unread Topics page, an ignore icon will also appear next to the forum a topic is in; clicking it will hide all topics in that forum when browsing Recent Unread Topics.
If you've accidentally ignored the wrong thing, or if you realise you want to read about insert subject here more than you previously thought, open your script manager's user script commands menu and click "Toggle ignored topic display".
Ignored topics will be displayed and clicking the appropriate ignore icon on them (you may have ignored a topic individually, by forum or both) will unignore them.
By default, this script also hides topic page number links in the Recent Unread Topics view and makes topic links go to the first unread post (when available).
This can be configured by changing the options at the top of the script.