This comes from the idiom “raise the bar.” The idiom “raise the bar” comes from sports like high jump thing and pole vaulting. There, to “raise the bar” means to increase the difficulty of succeeding. As an idiom, “raise the bar” similarly means to make the standard for success higher. If you set the bar low, then something is easy to achieve. If you set the bar high, success is harder to achieve. In your quote, it was really easy to find people guilty of witchcraft, but really hard for the accused person to prove that she was innocent.

It means that the amount and weight (or strength) of evidence required to prove guilt is very small. "Bar" here means level. For example, in some places, it is enough for one neighbor to accuse another of a crime or misdemeanor (e.g. insulting a holy person), for the accused to be considered guilty, as was the case of people being accused of witchcraft a few centuries ago in Europe. They were often killed as a result - no trial necessary.

Thank you.

Yes, the bar of guilt/innocence in your example is a variation of the idiomatic expression "set a high/low bar" or "lower the bar"—where "the bar" means "expected standard". So your example could be reinterpreted as: The standard of guilt is incredibly low. This means that it would be very easy to establish someone's guilt (and, in the following part, very difficult to demonstrate someone's innocence).