I just browsed through a few, including topics in which I know very well. These seem to be ~hour long talks on specialized topics. They seem comparable to a keynote address one might see at a scientific seminar. The topics are fairly niche, like an upper level college course. Picture of you just walked in on a grad level lecture of some random course you didn't know existed, like "The economic impact of health law and policy."

If you want to learn more, I'd recommend something like Khan academy, as it's designed for the general audience, and have topics that are a bit more introductory or general. Also, the production quality is better at keeping you engaged. The Gresham lectures are simply that: a lecture by a professor on a particular topic. They sometimes have a little bit of "death by PowerPoint" going on.

Although some lectures are interesting to everyone, like "life in the universe", many of the lectures have a less broad audience. Like unless you are into the history of technology you might not want to listen to an hour long lecture about Victorians' conception of space and time. It's very dry.

It's unsurprising that so few people are aware of these lectures. They aren't really marketed or aimed at the general public.