Flex containers and items are the building blocks of a Flexbox layout. Containers are the parent elements that hold the items, which can be any HTML element. To create a flex container, you need to set the display property to flex or inline-flex. The display property determines whether the container will be a block-level element or an inline element. Here's an example:

<div class="container">
  <div class="item">Item 1</div>
  <div class="item">Item 2</div>
  <div class="item">Item 3</div>
</div>