A place value system is a system of numerals that has a different value based on where it is located in the number. For example, take the number 1 in the decimal system. As 1, this numeral is only worth, well, 1. If we put a 0 next to it, it becomes worth ten, or 10.
This is important to understand as you learn about the binary, octal and hexadecimal systems, as they have the same numerals as the the decimal system, but each numeral has a different value according to it's place.