When I saw the first line of some Zig code, it was unusual so I thought about it and got an inkling of the idea behind it and its power:
const std = @import("std");
When I saw the second line and subsequent similar lines, I realized it was effing brilliant:
const os = std.os;
std and os here are names bound to types ... and you can have type variables and do compile-time manipulation and construction of types. This is confirmed in the manual when it talks about generic types being the return values of functions executed at compile time that take types (and/or other comptime values) as arguments -- while C++ templates are purportedly Turing complete, this is far more powerful in practice because it's vastly easier and more straightforward. Looking at code in the library like MultiArrayList--which implements AoS (array of structures) in the library rather than in the language--further confirms this.
So what's going on with those `std` and `os` bindings? The answer is given at the beginning of the Zig Language Reference (https://ziglang.org/documentation/master/) (which the OP apparently didn't read since it has a Hello World program that is an example of how to print): "The @import("std") function call creates a structure that represents the Zig Standard Library". So essentially, every source file represents an anonymous struct (all Zig structs are anonymous), the members of which are the top level declarations in the file--or rather, @import(filename) presents the file as such a struct, which can be assigned to a type variable like `std`. And `std.os` is in turn a type variable whose value is @import("os.zig") ... except that the actual value of `std.os` is computed based on the target machine. By turning files into comptime data structures containing all of the file's top level declarations, and having Zig code executable at comptime, immense power is achieved and one of the consequences of this is that zig running on any host is a complete cross compiler that can generate code for any target, using an appropriate target-specific version of the library. And it only took me a little bit of reading of docs and code to get my "aha" about how this works.