What does the "trash can" icon in the All Words section mean?

When the Sidebar is visible I can see in the All Words tab the following:

Does it mean that I accidentally marked the work “think” as deleted in one of the previous lessons? How can I “undelete” it and mark it as known?

Yes. It means you told Lingq to ignore that word. Just find the word in the lesson, click on it, and in the web version expand the definitions and scroll down. In mobile versions, just scroll to the bottom. Then select the level you want.