Make Motion More Natural Using a Bezier Curve Doubts

HTML-CSS
#1

Actually I cannot figure out the meaning of top:60%. What is it doing and also I am not sure what does position:fixed doing here.

Your code so far


<style>
  .balls {
    border-radius: 50%;
    position: fixed;  
    width: 50px;
    height: 50px;
    top: 60%;
    animation-name: jump;
    animation-duration: 2s;
    animation-iteration-count: infinite;
  }
  #red {
    background: red;
    left: 25%;
    animation-timing-function: linear;
  }
  #blue {
    background: blue;
    left: 50%;
    animation-timing-function: ease-out;
  }
  #green {
    background: green;
    left: 75%;
    animation-timing-function: cubic-bezier(0.69, 0.1, 1, 0.1);
  }

  @keyframes jump {
  
    50% {
      top: 10%;
    }
  }
</style>
<div class="balls" id="red"></div>
<div class="balls" id="blue"></div>
<div class="balls" id="green"></div>

Your browser information:

User Agent is: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:64.0) Gecko/20100101 Firefox/64.0.

#2