The Bubble Sort algorithm is very inefficient, but it is very easy to implement, and easy to explain:
1. Start at the top of the list and compare the first two numbers. If the first number is larger than the second number then reverse their positions.
2. Go down one position and compare the second number to the third number. As before, if the second number is larger than the third number then reverse their positions.
3. Continue down to the bottom of the list, comparing and reversing as necessary.
4. When you get to the bottom of the list, go back to the top and repeat steps 1 to 3. Do this repeatedly, going through the list, until you do one pass where none of the numbers need to change position. At this point the list is sorted.
It's inefficient, because if you have N numbers in the list and the smallest number is at the bottom of the list, then you'll have to go through the list N-1 times. Other algorithms are much more efficient, but more difficult to expain.