writers in English often do not explain everything to the reader. for example, in stories, the writer may not tell the time or place. Often the reader has to guess these things. this is called making inferences
making inferences is something called "reading between the lines." This mean you use the information in the text to guess other things about the text. It is often necessary to make inferences when you read. Something you need to guess about information the writer has not put there. Other times you make inferences all the time as they read.