I found this YAML syntax overview in Ansible documantation, to be fairly useful.
It says that double curly braces {{ variable }} are used to evaluate expressions. Whilst distinguishing a single curly braces (after a colon), that is used to declare a dictionary. For example:
satchmo: {name: Louis Armstrong, music: Jazz, instrument: Trumpet}
Also, take a look at Jinja Template Designer Documentation. Jinja template is rendered before YAML, which means it is evaluated prior to Ansible execution.
{% ... %} for Statements
{{ ... }} for Expressions to print to the template output
{# ... #} for Comments not included in the template output
# ... ## for Line Statements