Hi and welcome!
These abbreviations are for a slip stitch pattern:
YB, sl 1 purlwise, YF
YF, sl 1 purlwise, YB
Yarn to the back, slip one stitch purlwise, yarn to the front
Yarn to the front, slip one stitch purlwise, yarn to the back.
Front means the side facing you and back, the side away from you.