What does FS and SS stand for and how should I use lag time?

FS stands for Finish to Start. This indicates that the work order must finish before the next work order can start on your schedule. 

SS stands for Start to Start. This indicates that the work orders can appear on your schedule on the same start dates.

If you have a WO that has a duration of 5 schedule days and you don't want to wait until the end of those 5 days for the next work order to start, you can enter a lag time (example 3 days) so that the next work order will appear on the day that you need it to.