Can you tell me what is mild grade 1 spondylolisthesis?

Slight slip. Grade i spondylolisthesis is slippage of up to 25% of one vertebral body with regards to the adjacent one. A mild grade i slip would suggest that the displacement is on the lower side of the range. However, it is more of a descriptive term and does not specifically mean a particular percentage.
Spine slippage. Spondy means spine/ listhesis means to slip. A grade 1 (0ut of possible 4) is a forward slippage of one vertebrae over the one below. It can be a congenital/ develpmental problem or a result of degeneration.