We all know YAML is "a human friendly data serialization standard for all programming languages". In Drupal 8, the old .INFO files are now gone and have been replaced by YAML files - pronounced “yamel” (rhymes with camel). The following will show the tricks on strings,

Strings : Strings in YAML can be wrapped both in single and double quotes. In some cases, they can also be unquoted:

A string in YAML

'A singled-quoted string in YAML'

"A double-quoted string in YAML"