what is great fortitude or even fortitude specifically in rpg games, playing low magic age which has this

I find fortitude confusing, do not know whether or not it is good, bad whatever. I did look up what fortitude is and the explanation is more than confusing. I did also look up What happens if Fortitude sinks to 0? as well as https://rpg.stackexchange.com/search?q=fortitude . The game itself seems to be dnd-inspired, at least according to the developers and the people who are talking about it. Low Magic home page.

According to some it is more preferable that I talk or ask about fortitude in D&D. I haven’t ever played a paper-and-pen rpg so don’t really know.

From what I gathered from other people is that the game uses D&D 3.5e and uses ‘evolved OGL rules’ made by wizards of coast.