What is an SSL handshake?

Could you please explain what an SSL handshake is, and what role it plays in the security of online transactions? As a cryptocurrency and finance professional, I understand the importance of robust security measures, and I'm curious about how this specific process contributes to ensuring the safety of sensitive information during transactions. Additionally, I'd appreciate it if you could also touch upon any potential vulnerabilities associated with the SSL handshake and how they can be mitigated.