The word "stream" is an abstraction of a mechanism that enables the sending or receiving of an arbitrary quantity of bytes.
A river or stream is used as a metaphor.
When necessary, you can submit the data or accept it as it is.
Compare this to an array, which has a definite, known length, for instance.
Reading and writing to files, as well as receiving or transmitting data through an external connection, are examples of when streams are employed.
However, the word "stream" is ambiguous and doesn't specify the precise implementation.