In a nutshell - it's letting your
juice sit for a while.
Lots of
juice vendors make it fresh for you when ordered, and many of us are convinced that some time, and further mixing is needed for it to taste good. Some juice that uses alcohol based flavorings, in my opinion, needs a little time with the top off the bottle too!
But standard steeping is shaking it, and letting it sit for a week or more before you need to vape it!