Hover on a to-do (it can be from your current to-do list or from the history section) and click the three dots next to it.
From there, click "Save" to bookmark your todo.
Todos, subtasks and notes you take in a todo won't disappear once you bookmark them.
You can store all your important data by bookmarking them.
You can revisit them from the history section whenever you'd like.
FAQ
How do I access my bookmarked to-dos?
Go to the History Page, then click on the top right “Bookmarks” section, you can find your saved to-dos from there.