About parameter references

hey Happy to share my first question over here: When referring to parameters in Jenkins, why can I output them in single quotation marks.For example:
def str = “hello”
sh “echo ‘$str’”
The result is: hello ,Why not print$str

The Jenkins Pipeline domain specific language is derived from Groovy. It uses groovy string delimiters as described in:

A single quoted string does no variable expansion. A double quoted string expands variable references.