Think of the different columns as place value, the way you do in the tens system.
In the tens system, the first column (assuming no decimals!) represents units/ones - whatever digit there tells you how many ones you have (which is 10 to the power of 0)
The column to the left of that tells you how may tens (which is 10 to the power of 1), so whatever digit is there tells you how many tens there are.
The column to the left of that tells you how many hundreds (which is 10 to the power of 2), so whatever digit is there tells you how many hundreds there are.
The column to the left of that tells you how many thousands (which is 10 to the power of 3), so whatever digit is there tells you how many thousands there are.
and so on. It sounds really obvious in base 10, because it's how we name our number - so obviously 8362 means eight thousands, three hundreds, six tens, and two ones. You can only have up to the digit 9 in each column before you have to go to the next one.
But you can also label the columns in base 2 (binary) like that.
The first column tells you how many ones (which is 2 to the power of 0).
The column to the left of that tells you how many twos (which is 2 to the power of 1).
The column to the left of that tells you how many fours (which is 2 to the power of 2).
The column to the left of that tells you how many eights (which is 2 to the power of 3).
So if you have a number like 29 that you want to put into binary, you can make a little table where you label the place values (which probably won't format right on this, but just imagine 5 columns):
24=16 23=8 22=4 21=2 2^0=1
1 1 1 0 1
You need the biggest of those that you can fit into 29, which is 16, so you put a 1 in that column. That leaves 15 left. So you can fit an 8, so you put 1 in that column. That leaves 5. You can fit a 4 into that, so you put a 1 in that column as well, leaving you 1. So you can't fit any twos in that, so that column gets a 0, and the righthand column, the ones, gets a 1.
In other words, 29 is made up of 16+8+4+0+1, written in binary as 11101.
If you want to do in reverse, and take a binary number like 10010, you just plug it into the same columns, and add it up:
so that's one 24 = 16, no 23, no 22, one 21 = 2, and no 2^0. So 16+2 = 18, in base ten.
I'm sure someone else will have posted a better explanation by the time I've finished typing this!