you will be given what is called
a 'stem' word which you need to change in order to fill the gap.
This stem word can be altered into different word types like: nouns, verbs, adjectives, adverbs, gerunds (-ing verbs). This can be done adding prefixes (un-, dis-, a-, per- , pro- etc.) or suffixes )-able, -al, -ly, -ic, -ical, etc.).
For example: you are given the word LONG. You can change this to longest or longer, but also to length or lengthy. In a similar way, LIKE can be changed to likely/unlikely, dislike, liking, alike, likeness.
This exercise really tests your ability to connect different words back to one 'stem', and tie the meanings of words together.