CDO Tranches?

Does anyone know what this is referring to (from 3L’s must know list): 15) Make sure you can understand and can explain the difference between a cash arbitrage CDO and synthetic CDO and the different tranches (senior, mezzanine, equity). Specifically, the three tranches mentioned? I can’t seem to find anything on this…

end of ABS chapter. beginning of CDO section (schweser)

There it is, don’t know why I couldn’t find it. Thanks a ton!!