<div class="square"></div>

body{
  height: 100vh;
  display: flex;
  justify-content: center;
  align-items: center;
}
.square{
  width: 200px;
  height: 200px;
  background: #b74096;
  border-radius: 50px;
  animation: slide 2s infinite
}
@keyframes slide{
  0%{
    transform: translateX(-100px);
    opacity: 0
  }
}