Comment by LukeShu

The appendix does a pretty good job, but the TL;DR is: it will match any string containing 1 or more equal signs.

Slightly more verbosely, it will match [0-or-more bytes of anything] followed by [0-or-more bytes of anything] followed by [an equal sign] followed by [0-or-more bytes of anything]. The expensive part is that it can't decide where the first grouping of [0-or-more bytes of anything] starts and the second grouping begins. It doesn't matter where the division is, of course, but many regex engines use an exponential-time algorithm for that, even though an obvious liner-time algorithm exists (and pre-dates the exponential-time algorithm!).