Non breakable spaces are such where there is no division of words between two lines. Use it for example before one character prepositions, or as the separator of thousands in numbers or phone numbers.
It may be inserted either by pressing shortcut keys Ctrl + Shift + Space or Alt + Space, or in the Functions menu (in features section for text boxes), it is where the 
button is located. The editor maintains fixed spaces that are inserted from an external program.
If you want to automatically insert a fixed spaces in front of all single-character prepositions and conjunctions, press 
, again in functions section of text boxes.