CSS

css after

.parent-div {
   position: relative;
   width: 100px;
   height: 100px;
   background: #ffffff;
   border: 1px solid #000000;
}
.div:after {
  content: "";
  width: 20px;
  height: 20px;
  background: blue;
  position: absolute;
  top: 45%;
  left: 45%;
}

after in css

span[data-descr] {
  position: relative;
  text-decoration: underline;
  color: #00F;
  cursor: help;
}

span[data-descr]:hover::after,
span[data-descr]:focus::after {
  content: attr(data-descr);
  position: absolute;
  left: 0;
  top: 24px;
  min-width: 200px;
  border: 1px #aaaaaa solid;
  border-radius: 10px;
  background-color: #ffffcc;
  padding: 12px;
  color: #000000;
  font-size: 14px;
  z-index: 1;
}

::after

a::after {
  content: "whatever content you want to display";
  /*css declaration*/
}