Place value describes how the position of a digit in a number determines how much it is worth. In 4,372, the 4 is worth 4,000, the 3 is worth 300, the 7 is worth 70 and the 2 is just 2 — same digit, completely different values depending on where it sits.
The same idea extends to decimals. In 3.47, the digit after the decimal point is the tenths column, and the next is the hundredths column: $$3.47 = 3 + \frac{4}{10} + \frac{7}{100}$$
Place value is the foundation for adding, subtracting and rounding correctly, so it's worth being really solid on it. A common slip is confusing numbers where a zero acts as a placeholder — for example, mixing up 4,072 and 4,720. The zero isn't meaningless; it holds a column so all the other digits stay in the right place.