What do the marks on the head of a bolt mean?

They represent the hardness of the bolt. No marks indicates the least hard, three marks is medium hardness and bolts with six marks are the hardest.

Check your state and local codes before starting any project. Follow all safety precautions. Information in this document has been furnished by the North American Retail Hardware Association (NRHA) and associated contributors. Every effort has been made to ensure accuracy and safety. Neither NRHA, any contributor nor the retailer can be held responsible for damages or injuries resulting from the use of the information in this document.