Content in Twig templates can translate content either in small sets by using the t or trans filter, or the {% trans %} block. For instance, you can run text through the translate function using either of the following:

{{ 'Hello world' | t }} 
{{ 'Hello world' | trans }} 

Placing output within a {% trans %} block runs text through the translation function, including handling variables. For example:

{% trans %} 
  Submitted by {{ author.username }} on {{ node.created }} 
{% endtrans %} 

This is the same as running t('Submitted by @username on @created') and passing the appropriate variables. The default variable handling is to escape the values, but you are also able to avoid this escaping or wrapping it in an <em> tag by using filters. For example:

{% trans %} 
  Submitted by {{ author.username | passthrough }} on {{ node.created | passthrough }} 
{% endtrans %} 

This is the same as running t('Submitted by !username on !created'). Similarly:

{% trans %} 
  Submitted by {{ author.username | placeholder }} on {{ node.created | placeholder }} 
{% endtrans %} 

This is the same as running t('Submitted by %username on %created').