What does it mean that the Hong Kong and Macao Pass endorsement type is once a year?

Once a year does not mean that you can only go once a year, but the endorsement is valid for one year. If you travel to and from Hong Kong and Macao within one year, you can apply for an endorsement again, and the number of applications is unlimited. If you don't go within one year, the endorsement will be invalid and you can apply again.

At present, residents of 49 cities can come to Hong Kong for "individual tours". Cities that offer "individual tours" are:

References:

Baidu encyclopedia? Web page link