Without clear
div1
div2 - Notice that the div2 element is after div1, in the HTML code. However, since div1 is floated to the left, this happens: the text in div2 is floated around div1, and div2 surrounds the whole thing.
Using clear
div3
div4 - Using clear moves div4 down below the floated div3. The value "left" clears elements floated to the left. You can also clear "right" and "both".