Serialization of object is very old technique and it does not depend on .NET.
When an object is serialized, the data stored in the object (including reference to other objects) is converted into the stream of bytes that can be more easily stored into the database or other file system.
This serialized data can be utilized in future to rebuild the object and this process is known as deserialization.
Before .NET it was possible to serialized/deserialized the object but it required more manual coding.
Since, object is base data type in .NET, serialization/deserialization functionality is built in to the Framework.