What does one red line through the weapon mean?
Solution 1:
According to the New Player Help page on the Salt and Sanctuary Wiki, it indicates that the weapon should be used two-handed (emphasis mine):
If you see weapons or armor with two red lines forming an X, it means you don't have the requirements to use the item. If you see only one red line (over weapons) it means you must two-hand the weapon to use it effectively.
This link describes how to switch to two-handed in your loadout.