To "think it over" means to think about something that you have to make a decision on. In the example above, the job applicant has to "think over" whether to take the job.

This expression almost always uses "it":

Let me think it over.

If you don't use "it", the word order changes:

Have you thought over my offer?

A more formal word that means something similar to "think it over" is "consider". You would write this if you were offering someone a job in writing:

Please consider our offer and let us know what your decision is by next Thursday, March 10th.