As Saturn Dreams mentioned, if you want it to have internet connection (almost) anywhere, you need cellular data service (Verizon/AT&T/Sprint/T-Mobile). You need a tablet has a cellular radio for a specific carrier plus a data plan on that carrier.
Another way to go is a discrete WWAN (wireless wide area network, i.e. cellular) adapter that connects to multiple WiFi devices. WiFi is short range (300 feet line of site to internet connected radio). WWAN is everywhere there are robust wireless data networks.
WWAN adapters are usually called MiFi or mobile hotspot devices. Many smartphones offer hotspot service as an add-on service for extra cash. All tablets have WiFi of some stripe. A MiFi or hotspot device is something you would probably want to get from your cell phone provider and will require a data plan, but prepaid devices exist too. They send and receive WWAN data over cellular networks on one end and WiFi data to mobile devices like tablets, laptops, mobile phones, etc. on the other end.
Basically, to connect to the internet, you need a path. In your home, you most likely have a cable or DSL modem that may or may not have a WiFi router built in. Assuming it has WiFi, you can connect a tablet at home. The path is from tablet to WiFi router to internet via your modem to the internet provider. If not, you will need a path from your tablet to the internet at home, too. A wireless router attached to your modem or a MiFi/hotspot device attached to your cellular provider.
If you want to take your tablet away from home, you need it to connect to another path. Someone else’s WiFi (public, hotel, etc.) or have a tablet with a WWAN radio to connect to Verizon, AT&T, etc. Or have a hotspot device that connects to the same service on one end and your tablet on the other.
The WWAN/cellular radio, whether it’s built into the tablet or a discrete device that can provide connection to multiple WiFi devices, is going to cost money up front and money in a service agreement, whether monthly or prepaid. Built in to the tablet is more convenient, wireless hotspot is more versatile as you can use it with anything WiFi capable.
Hopefully, this has been more informative than confusing.