Tooltip

<div class="tooltip">Hover me</div>

.tooltip {
  position: relative;
  display: inline-block;
  padding: 8px 16px;
  background: #333;
  color: white;
  border-radius: 4px;
  cursor: pointer;
}
.tooltip::after {
  content: "This is a tooltip";
  position: absolute;
  bottom: 125%;
  left: 50%;
  transform: translateX(-50%);
  background: #333;
  color: white;
  padding: 8px 12px;
  border-radius: 4px;
  font-size: 14px;
  white-space: nowrap;
  opacity: 0;
  visibility: hidden;
  transition: opacity 0.3s, visibility 0.3s;
  z-index: 1;
}
.tooltip::before {
  content: "";
  position: absolute;
  bottom: 115%;
  left: 50%;
  transform: translateX(-50%);
  border: 5px solid transparent;
  border-top-color: #333;
  opacity: 0;
  visibility: hidden;
  transition: opacity 0.3s, visibility 0.3s;
}
.tooltip:hover::after,
.tooltip:hover::before {
  opacity: 1;
  visibility: visible;
}