Understanding Signals: Data Basics and Job Change Handling

Hi, I am having trouble understanding something about signals, and the docs didn't help. What's the base line of signals? The data I have in my CRM or the data the person has when I add them to Signals? Especially when discussing job changes. Also, with job changes, what happens if someone ads a current job on top of the one they have right now? will Signals make sure to check that the current job has ended on their profile?