<div class="fixed-thing"></div>
<div class="other-thing"></div>
<div class="other-thing"></div>
<div class="other-thing"></div>
<div class="other-thing"></div>
<div class="other-thing"></div>
<div class="other-thing"></div>
<div class="other-thing"></div>
<div class="other-thing"></div>
<div class="other-thing"></div>
<div class="other-thing"></div>

.fixed-thing {
  background: orange;
  height: 50px;
  position: fixed;
  width: 100%;
}

.other-thing {
  background: #eaeaea;
  height: 500px;
  width: 100%;
}