Ancestral traumas are traumas that are shared between any two (or more) family members, either because the trauma happened to them both/all directly or because of some close connection they have... and you are physiologically and emotionally (whether you like it or not) quite close to your family, obviously, so ancestral traumas can really stack up in your space.
Let's look at an example. If Jake is wanting to create a healthy, marriage-bound relationship, and he's in the process of clearing out old traumas that have been getting in his way, he may find that the trauma of his sister's divorce may have as much of an effect on him - but in a different way - as his parents' divorce which is more noticeably traumatic, simply because he loves his sister! He doesn't know how to process the fact that he resents and can also relate to her ex. Or he might be registering a trauma that originated from his great-grandparents who were happily married, but during the great depression experienced a lot of misery and financial instability... The message he registers, based on whatever that family story contains, is that even good marriages have to pay the price somewhere, that no one can have it all.
Ancestral traumas can run the gamut. We all have them, because we all have families who share our DNA and therefore are quite linked to us all the time. (Read about the non-local nature of DNA for more tidbits about how we send and receive healing through these biological connections). The benefit to this linkage is that we are sending our entire family line little messages of healing - little shortcuts towards health - every time we heal ourselves. When we heal ourselves, we truly help heal everyone we're connected to... which ultimately iseveryone.