Float is an often misunderstood CSS property, however when used properly it can be a very useful technique for implementing CSS layouts.
Quoting the CSS standards guide at the w3c website:
A float is a box that is shifted to the left or right on the current line.
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This property specifies whether a box should float to the left, right, or not at all.
The float property pulls an element out of the normal flow and places it in either the left or right ends of the current line inside its containing block element. Other elements will then wrap around the floated element, providing they have enough space to do so (if the cumulative width of the elements is too wide, they will slide to the next line).