In the olden days, if you wanted to know what was going on, you had to read at least a synopsis. If you wanted to get the details, you’d need to review the full libretto and listen to some highlights. Now, you can walk into an opera house knowing nothing about the work being performed, with full confidence that you’ll understand the plot.
As for the details—that depends on the quality of supertitles, which is far from consistent. Here are some types of supertitles I’ve encountered in the wild."