To illustrate the idea of keywords I will use an example question from IELTS Listening exam section.
Before the recording starts you will be given some time to look at the questions. In IELTS, the speaker will let you know which questions you are going to be answering. You are encouraged to scan-read through these questions to have an idea of the information you are about to be presented with after the recording starts to play. As you look through the questions, you should underline the most important parts, otherwise called keywords.
Below is an example question with proper underlining:
Let’s go through the underlined fragments one-by-one.
Two – although this part will be written in bold, capital letters, it is recommended to underline it nonetheless. It’s self-explanatory — you will have to choose two letters.
John’s – this is underlined because the recording might involve more than one student, and we need to know professor’s opinion about John’s paper, not somebody else’s work.
Didn’t like – it is possible that all of the aspects above will be mentioned. But to answer this question we are only interested in those the professor is not very happy about. It is a common mistake to simply mark the answer a student hears mentioned in the recording. Additionally, you might want to put some symbol above “didn’t” that has a negative meaning, such as a minus, a sad face, a downward arrow (-; ↓; ☹). This will make the search easier.
Extra info for the Reading section: it is recommended to underline dates (e.g. 7th Nobemver 1990, the 21st of December) and any proper names (e.g. Mark, Luxembourg, Laika) as both of them are very easy to spot in the text and are unlikely to be paraphrased.