What does colon (:) mean for getopts?

I am trying to parse options for my script with getopts and I decided that it would be best to read up on it in the POSIX standard, as it's usually extremely helpful. The *DESCRIPTION* section is fairly ambiguous in regard to the use of colon (:) in *optstring*, so I looked up how people use : with getopts on the internet and now I'm confused. What does colon do in commands like getopts abc:d name and how am I supposed to know just by reading the standard?