What does thecolon minus do in a shell?

Q: I am looking at a script written for the runscript shell. I have a line as
follows:

if [[ ${RC_INTERACTIVE:-yes} == "yes" ]] ; then

I would like to know what the :- (colon minus) operator that follows the
variable name does.

A: Like in a normal (POSIX like) shell - "info bash" says:

${PARAMETER:-WORD}'
If PARAMETER is unset or null, the expansion of WORD is
substituted. Otherwise, the value of PARAMETER is substituted.