In positional notation, the same digit is worth different amounts in different places. The 3 in 300 is worth three hundred; the 3 in 30 is worth thirty.
Every position has a weight. Number the positions from the right, starting at 0. In base b, position p has weight bᵖ. The value of the whole number is each digit times its position's weight, all added together.
The idea comes from bundling. Counting matchsticks in tens, you group 10 loose sticks into a bundle and 10 bundles into a crate, and each column records how many of that size you have. Bundle in twos instead and you get binary; bundle in sixteens and you get hexadecimal.
Why it matters: one short rule handles every base. Converting to decimal, adding with carries, and multiplying by the base (adding a 0 on the right) all work the same way, whatever the radix is.
Zeros earn their keep here. A 0 inside a number holds a place open, which is why 302 and 32 are different numbers.