Place Value
Place value is fundamental to understanding maths and the number system. It is also one of the most misunderstood subjects. If children don't understand that a digit has a different value depending on it's position in a number then they really struggle to understand maths. Numicon is a great resource to help children understand place value as are dienes and place value counters.
It's really important for children to see that the pattern in the numbers just carries on forever. Once they understand HTU they can see that this pattern is simply repeated but this next group of 3 are actually in the thousands, the next group of 3 in the millions etc.
Once they understand this concept they really love seeing how large a number they can read and start googling and playing with very large numbers. Putting in the commas every 3 numbers really helps.
It's really important for children to also understand that a 2 in the units is only worth 2 but if its in the tens place then it doesn't have the value 2 anymore but is now worth 20.