Toe in GPS

Hi everyone!

I am doing a project extracting data from RINEX files and although I've read a lot of documents, I'm still confused about some points and really need your help:
1) What is exactly toe ( reference time for the ephemeris )? and what is the difference between toe and tow in navigation message?
2) What is week crossover and why we have to + or - 604800 s to modify time of transmission ?

Thanks for a swift reply.