Beginner - cast on and pattern help!

The pattern states “Cast on 37 stitches (or any combination of 3 + 1)” What does the 3 + 1 mean??

It means the stitch pattern needs a multiple of 3 sts plus one. 37 is 3x12+1, and if you wanted to make this wider or narrower (or using a different yarn and needle size and need to adjust the sts), you would need to add or reduce the sts in increments of 3.