Spacing effect is a trick to remember things better by giving them space and time. It means that if you learn something, and then you wait a little bit, then learn it again, and then wait again, it becomes easier to remember for a long time.
Imagine you have to remember the letters of the alphabet. Instead of trying to remember all 26 letters at once, you can break it into small parts. For example, you can memorize A, B, and C today, and then tomorrow you can learn D, E, and F, and so on.
Your brain needs time to rest and work on memorizing what you learned last time. This way, you give your brain enough space and time to learn and memorize things better, without overwhelming it.
So, the spacing effect tells us that people remember things better when they learn small pieces of information over time, rather than trying to learn everything at once.