Answer: summarizing ideas.
It is impossible to write a good summary of any type of information unless you have first read or watched the whole text, video or performance. In order to write a summary, you need to be familiar will all the main ideas of the work. You also need to be familiar with the details, as you need to decide which details are worth including in your summary. A summary is a useful tool when we want to be able to recall the important points of a work later on.