Non-breaking spaces are used to keep words and numbers (in every possible combination) that are separated by a space, together (that is: on the same line).
Non-breaking space are represented as a centered dot・in normal view (in the translation editor) and as a ⨆ in Show invisible characters mode (activated by clicking on the blue pilcrow sign ¶ above the Translation Editor). Non-breaking space are also shown in some dialogue boxes (Find and Replace, Add terms …).
Inserting Non-breaking Spaces Before Units
Inserting Non-breaking Spaces before Numbers
Automatically inserting non-breaking spaces before units