Fortitude is physical endurance/resilience
Fortitude - at least in the sense used in 3rd and 4th edition Dungeons & Dragons - is a character's physical endurance/resilience to harmful effects. In 3e/3.5e D&D, characters have a Fortitude Saving Throw bonus, which is added to their rolls to resist appropriate effects; in 4e, characters have a Fortitude Defence, which is a number (like Armour Class) that someone trying to apply a harmful effect has to overcome in order to successfully affect them. As the 3.5e SRD describes:
These saves measure your ability to stand up to physical punishment or attacks against your vitality and health.
It is unfortunate that D&D's designers chose to use the term "fortitude", given that the normal English meaning of the word refers to mental resilience, but it's the obsolete definition meaning "strength" which they're invoking here.
The kind of effects that would be resisted by a character's Fortitude include (but are not limited to) things like:
- being poisoned
- being afflicted by a disease
- being nauseated by a foul smell
- being knocked down by a blast of wind
- being petrified (turned to stone) by a spell or monster
This is contrast to the character's Reflex save/defence, which is how good they are at nimbly evading some harmful effects, and their Willpower save/defence, which is a measure of their mental strength against effects like charms and illusions.
Great Fortitude, specifically, is a feat which improves the character's Fortitude - in 3e it grants a +2 bonus to Fortitude saving throws. I'm not sure if an equivalent exists in 4e, as I'm far less familiar with that game.
Low Magic Age looks like it is mostly derived from 3rd edition D&D rules, but from the screenshots it looks as if it might have borrowed the 4e mechanic of replacing Saving Throws with Defences - I'm not interested in buying the game to find out exactly how it works.