There are two ways to access internet on your Smart TV.
1. Through Ethernet cable meaning your regular wire based internet.
2. Through Wi-Fi router.
Now, the Wi-Fi ready, meaning you can access the internet through Wi-Fi router but you need a external device i.e. dongle to access it that needs to be purchased separately and connected to the TV.
In build Wi-Fi on the other hand means a TV that comes with an in build mechanism of Wi-Fi access and does not require an external device to access it, Hope that helps you understand the basic difference between the two.