Answer 1

The following article might be able to help you familiarize yourself with streams some more:

https://www.cprogramming.com/tutorial/c++-iostreams.html

<iostream> itself is a library that enables you to use streams to perform basic input and output in your program. Without it, you couldn't ask a user of your program to enter some values via keyboard, or print any characters to the screen.

A library is basically a file containing itself code that is then included in your program, so you can make use of the functions provided in that file.
By separating your program in such libraries, you can structurize your code, make it more flexible and reusable, and you don't end up with a single file consisting of thousands of lines of code that you have to go through.

std itself is just an identifier for a namespace, so a better question would be what a namespace is:

https://www.learncpp.com/cpp-tutorial/2-9-naming-collisions-and-an-introduction-to-namespaces/