Progression in Godfall will often be closed off behind items called Sigils. A mission to advance the story may have a certain Sigil requirement, and you won’t be able to play through it until you gather up that number of Sigils.
Thankfully, finding Sigils is actually very easy. Every mission in the game will reward a certain amount of Sigils upon completion. You will earn those Sigils upon the completion of the mission objective.
Some missions, like the Hunts, will reward a number of Sigils and can be played through numerous times to farm them. For example, the Alpha Rancor Hunt, which takes place in the Crimson Glades on earth and is accessible quite early in the game, will reward 2 Earth Sigils upon completion.
These Hunt missions can be played over and over again, reward Sigils each time, so if you are out of other options they are a great way to earn them. They will also reward other important resources, and will have secondary objectives that you can complete to get other rewards, making them a pretty solid way to grind items in the game.