Subtraction involves removing objects from a group to determine how many remain. It can be represented by equations such as 5-2=3, meaning two objects were removed from five, leaving three. Word problems use subtraction to determine amounts remaining after some objects are taken away or used. For example, if a teacher started with 84 gifts and gave 67 to students, subtraction can be used to find the 14 gifts remaining with the teacher. Key words like 'take away', 'left', and 'remain' indicate when to use subtraction in word problems.