Padding is the space left empty between an element's edge and what's in it, so a text or a barcode doesn't touch the border. Each part of an element has its own: a text, a barcode, a picture, a grid, a pattern and a line can each keep a different distance.

The padding control
Top, Left, Right, Bottom	The space on each side, in the unit you work in.
	All the same. The chain is broken while you set each side on its own; click it to link the four, and what you type in one goes to all of them. Click again to unlink them.
	Shows how the four are measured: the ruler, Fixed values, for measurements; %, Percent, for a share of the element - left and right of its width, top and bottom of its height - so the space grows and shrinks with it. Click it to switch.

Padding that adds up to more than the element leaves nothing to draw in: the part disappears until you make the element bigger or the padding smaller.

The same four boxes turn up elsewhere with other meanings: the Page margins of the Layout panel (in measurements only), an element's Position, and the crop of a picture, in pixels.