Repeating fields have a long and somewhat controversial history in FileMaker. If you want to start a heated discussion with another FileMaker developer, start one about repeating fields! Some developers love them, some hate them and then there are some like me, who appreciate them as another tool in the belt to be used to accomplish functions that would be hard to do without them.
If you started building FileMaker applications from scratch in more recent versions of the product you may not be very familiar with repeating fields and probably have never used them. You may even have an understanding of what they are but have never found a need to use them. If you've been using FileMaker for a long time or have taken over a legacy solution, chances are you've had to deal with them.