How does the "position: sticky;" property work?

I want to make the navigation bar stick to the top of the viewport once a user scrolls the page, but it's not working and I have no clue why. If you can please help, here is my HTML and CSS code:

.container {
  min-height: 300vh;
}
.nav-selections {
  text-transform: uppercase;
  letter-spacing: 5px;
  font: 18px "lato",sans-serif;
  display: inline-block;
  text-decoration: none;
  color: white;
  padding: 18px;
  float: right;
  margin-left: 50px;
  transition: 1.5s;
}

.nav-selections:hover{
  transition: 1.5s;
  color: black;
}

ul {
  background-color: #B79b58;
  overflow: auto;
}

li {
  list-style-type: none;
}

<main class="container">
  <nav style="position: sticky; position: -webkit-sticky;">
    <ul align="left">
      <li><a href="#/contact" class="nav-selections" style="margin-right:35px;">Contact</a></li>
      <li><a href="#/about" class="nav-selections">About</a></li>
      <li><a href="#/products" class="nav-selections">Products</a></li>
      <li><a href="#" class="nav-selections">Home</a></li>
    </ul>
  </nav>
</main>

Solution

Sticky positioning is a hybrid of relative and fixed positioning. The element is treated as relative positioned until it crosses a specified threshold, at which point it is treated as fixed positioned.
...
You must specify a threshold with at least one of top, right, bottom, or left for sticky positioning to behave as expected. Otherwise, it will be indistinguishable from relative positioning. [source: MDN]

So in your example, you have to define the position where it should stick in the end by using the top property.

html, body {
  height: 200%;
}

nav {
  position: sticky;
  position: -webkit-sticky;
  top: 0; /* required */
}

.nav-selections {
  text-transform: uppercase;
  letter-spacing: 5px;
  font: 18px "lato", sans-serif;
  display: inline-block;
  text-decoration: none;
  color: white;
  padding: 18px;
  float: right;
  margin-left: 50px;
  transition: 1.5s;
}

.nav-selections:hover {
  transition: 1.5s;
  color: black;
}

ul {
  background-color: #B79b58;
  overflow: auto;
}

li {
  list-style-type: none;
}

<nav>
  <ul align="left">
    <li><a href="#/contact" class="nav-selections" style="margin-right:35px;">Contact</a></li>
    <li><a href="#/about" class="nav-selections">About</a></li>
    <li><a href="#/products" class="nav-selections">Products</a></li>
    <li><a href="#" class="nav-selections">Home</a></li>
  </ul>
</nav>