Default Gateway
Problem
Your laptop wants to reach:
8.8.8.8
That's outside its subnet.
Your laptop cannot deliver the frame directly to Google's machine because Google isn't on the local Ethernet network.
So:
Who should receive the packet first?
Solution — Default Gateway
The laptop sends the packet to its router.
Laptop → Router → Internet → 8.8.8.8
The IP destination remains 8.8.8.8, but the local Ethernet frame is addressed to the router.
Core idea: the default gateway is where a host sends packets when it doesn't know a more specific local route.