container class in bootstrap

Containers
Containers are the most basic layout element in Bootstrap and are required when using our default grid system. Choose from a responsive, fixed-width container (meaning its max-width changes at each breakpoint) or fluid-width (meaning it’s 100% wide all the time).
While containers can be nested, most layouts do not require a nested container.

Syntax:
<div class="container">
  <!-- Content here -->
</div>

Our default .container class is a responsive, fixed-width container, meaning its max-width changes at each breakpoint.

<div class="container">
  <!-- Content here -->
</div>

meaning it’s 100% wide all the time

In Bootstrap, container is used to set the content's margins dealing
with the responsive behaviors of your layout. It contains the row elements
and the row elements are the container of columns (known as grid system).

The container class is used to create boxed content.

<div class="container-sm">100% wide until small breakpoint</div>
<div class="container-md">100% wide until medium breakpoint</div>
<div class="container-lg">100% wide until large breakpoint</div>
<div class="container-xl">100% wide until extra large breakpoint</div>