.container has one fixed width for each screen size in bootstrap (xs,sm,md,lg);

.container-fluid expands to fill the available width.

@media (min-width: 568px) {
  .container {
    width: 550px;
  }
}
@media (min-width: 992px) {
  .container {
    width: 970px;
  }
}
@media (min-width: 1200px) {
  .container {
    width: 1170px;
  }
}

Depending on the width of the viewport that the webpage is being viewed on, the container class gives its div a specific fixed width.

Your .container-fluid element, on the other hand, will constantly resize as you make even the smallest changes to your browser width.