For absolutely positioned elements, the bottom property sets the bottom edge of an element to a unit above/below the bottom edge of its containing element. For relatively positioned elements, the bottom property sets the bottom edge of an element to a unit above/below its normal position. Negative values are allowed.
Note: the way an element moves on screen when you apply a positive 'bottom' value may seem counterintuitive. It moves up on the screen when given a positive value because the browser increases the distance between the bottom of the element and some reference point. If in doubt, just look at the direction of the property icon. The icon depicts the direction the element will move on the page as you increase the value for 'bottom'.
e.g. '75' would move an element 75 pixels below the bottom edge of its parent element (if the element is absolutely positioned), or 75px below it's normal position (if the element is relatively positioned)