The terms "online" and "offline" have specific meanings in regard to computer technology and telecommunications. In general, "online" indicates a state of connectivity, while "offline" indicates a disconnected state. The concepts have however been extended from their computing and telecommunication meanings into the area of human interaction and conversation, such that even offline can be used in contrast to the common usage of online.
When you are online, it means you are connected to a computer network, and for most personal users this network is the Internet. In addition, it can also mean that you are using your email or instant messaging program that allows you to communicate with another person.
While online, you can do anything on the Internet, from sending and receiving emails or instant messages, engage in a video call with someone, use social network tools, work and even play games that require an Internet connection to work.
You can browse while you are offline as well, but you only able to view local copies of web pages that you had open during your last time online. These copies are stored locally in your local storage and can often be viewed even without an Internet connection. If you are considered “offline” on a smartphone, you can still do regular phone things like engaging in voice calls, texting with other people and playing games that don't require an Internet connection.
You are unable to browse the Internet on your smartphone or tablet while offline. Likewise, you cannot play any game that requires you be online, and you cannot stream music or video from the Internet while offline, but you can play anything that's stored on your device since it's stored in your device's local memory.