What exactly is progressive metal and what are the recs?

I'm very much a metal noob who got into the genre only 3 years ago. Only recently started to get into male-fronted ones and I'm still mostly limited to bands that are not that old. Rarely founded before 1995.

My current tastes include bands that are neither too mainstream nor too extreme. To make it clearer, I adore Nightwish, Jinjer, Epica, Within Temptation, Seven Spires, Nervosa, Arch Enemy, Ignea, Eluveitie, Fleshgod Apocalypse and stuff like that.

I guess that the only hint of what prog is that I heard is Dream Theater and I guess Gojira (which I'm trying to know better) and I think I'm missing out because prog musicians are apparently some of the best according to metal subreddits and I don't know where to start.

There's Opeth, but apparently, they went by totally different direction that's barely metal anymore if at all. But he's seen as one of the biggest geniuses.

I mostly need to know what prog is since the descriptions I've heard are too unclear to me and I need very simplistic explanations because my knowledge of music theory is nonexistent.