Margin demo

No styles are applied to either of these paragraphs

However, clearly there is space between them.

Margin demo with borders turned on

This is the same arrangement of a heading and two paragraphs but now the borders are visible, and you can see how the default margins creates space between the elements.

Margins push an element away from elements it would otherwise touch.

Margin demo with borders on, but without margins

Here the margins are set to 0 (zero), so the elements' borders actually touch.

Be aware that almost every element has margins by default, and you will often want to eliminate or adjust the default margin setting to get the effect you want.