Validation - Data Ranges
Validation is when you limit the possible data that can be put into a cell on a spreadsheet (or another data holding structure such as a Variable in programming). Validation stops people entering data that is "not allowed" .
Validating using data ranges is one of the most common ways to limit data, it is also one of the easiest.
For example, if you are issuing driving licences (in the UK) you might have a validation range on the age cell of 17 to 100 (the high end is debatable but it doesn't matter for the example).