Reading skill
Gist means the general meaning of a text. Reading for gist is reading quickly to get the main idea, without understanding every word.
In IELTS you do not have time to read every word. If you stop on each new word, you run out of time and you lose the meaning. Reading for gist trains you to move fast and still understand the main idea.
You do not need 100 percent of the words. Often a small number of keywords carries most of the meaning. If you understand those keywords, you understand the gist, and that is usually enough to answer many questions.