Hello, i failed to understand what the scale on the y-axis means, please explain, and may you comment the features or the trends that the graph is showing, and what caused those features or trends, thanks a lot

FRED ~ - Unit Labor Costs in Manufacturing, U.S. Dollar Basis, in Japan (DISCONTINUED) 160 140 120 100 Index 2002=100 80 60 40 20 0 1955 1960 1965 1980 1985 1990 1995 2000 2005