Both items on the Check ▾ dropdown work on the word your text cursor is in — you don’t have to select it first. Click into a word in TextEdit, Notes, Mail, Safari, Word, Pages or another app, then choose:
- Look up
- Opens the word in the macOS Dictionary app.
- Thesaurus / synonyms
- Shows the Word Information popup for the word, where you can see its synonyms.
If you’ve selected some text, the selection is used instead. In apps that don’t share their text — such as some PDF viewers — Readable uses the word you were pointing at just before you opened the Check menu.