It is explained at the very end of their specific paragraphs:
https://docs.djangoproject.com/en/dev/topics/i18n/translation/#trans-template-tag
{% trans %}
also supports contextual markers using the context keyword:{% trans "May" context "month name" %}
https://docs.djangoproject.com/en/dev/topics/i18n/translation/#blocktrans-template-tag
{% blocktrans %}
also supports contextual markers using the context keyword:{% blocktrans with name=user.username context "greeting" %}Hi {{ name }}{% endblocktrans %}