This selector means that the style applies only to h1 tags directly following another tag. If you replaced * with p it would affect any h1 following a p tag.
h1 {
margin-top: 0; /* Sets our H1 to have no margin - meaning our 1st one */
}
* + h1 {
margin-top: 1em; /* Now all the rest of the H1 tags that follow other items will have a top-margin */
}