Your standard login credentials including password starts the process, but the physical key is required for access to the account. Standard password verification is based on personal knowledge, typically a sequence of letters, numbers or symbols. However FIDO U2F verification adds a detail and requirement based on what a person physically has in their possession, the FIDO U2F security key. It creates an added layer of security, an encrypted device unable to be hacked, hijacked, phished, or spoofed. It's a more fool-proof way of confirming identity and best described by Google;
“Security Keys are mandatory at Google, they provide superior protection against phishing not possible with many alternative two factor authentication solutions.” - Christiaan Brand, Product Manager for Security and Identity at Google, eWeek.