Sometimes, registering a theme hook and a template for outputting some data can be overkill. Imagine that all you have is a string you need to wrap in a <span> tag or something. In this case, you can use the #markup property, which specifies that the array provides directly the HTML string that needs to be output. Note, however, that the provided HTML string is run through \Drupal\Component\Utility\Xss::filterAdmin for sanitization (mostly, XSS protection). This is perfectly fine because if the HTML you are trying to include here is stripped out, it's a good indication that you are overusing the #markup property and should instead be registering a theme hook.
Going a bit further than just simple markup is the #plain_text property ...
