So words are more commonly used than others. Without knowing exactly which word you want to know about, I cannot tell you how often it is used. If you are using an IELTS book written by someone who is not a native speaker of English, that person might not be familiar with how frequently each word is used, so they might not know which words are the most important. It would be great if each word came with a number. For example, a word with number 1 could be very commonly used, and a word with a 5 could be very rarely used. English is difficult! Yes, most English words have more than one meaning and the meanings can be completely different from each other. One thing that I recommend is that you read a lot of writing written by native users of the language. This way, you will see the word in context. If you don't know the meaning of the word in a specific context, mark it and look it up later. Use an English-English dictionary for learners (preferably one with example sentences), such as this one: http://www.learnersdictionary.com/ Good luck!

Vocabulary is the group of words that a person or group of people knows how to use. Your vocabulary is all the words you know and use regularly.

a lot of words are used manly in books but some people use them but that is very unlikely