What is standardization in computer science?
A standard for a programming language is a document defining the syntax and semantics for that language. Usually, for real-world languages, this document involves a (hopefully) precise description in intuitive terms, rather than a formal semantics, written in mathematical terms.

Still, this document acts as a contract between the programmer and the language implementation (usually a compiler/interpreter, plus some "standard libraries").

The programmer knows that, if they stick to what the standard mandates, they will get back the intended result from the implementation. Vice versa, the implementor of the language can assume that the programmer used no other features than those mandated. Often, the implementor will also provide non-standard features (additional APIs, additional language constructs, OS-specific libraries, etc.).

Note that the standard does not only involve describing the APIs in the standard library. In the case of the C language, as mentioned by the OP, the standard defines among others: the syntax, the type system, the memory model, the statements, the variable declarations, etc.

For instance, what happens if we write int *x; *x=0;? The ISO standard answers that: an implementation printing mooo! on screen is perfectly conforming (as any other implementation is).

So, summing up, a standard is the authoritative document which answers questions like "is running code C guaranteed to have behavior B?".

Most standards are written by a committee made of the most prominent organizations which developed / are developing the most widespread implementations. They vote on which features should be in or out, until they reach an agreement on what should be the standard definition. Some languages are revised every few years, so to include more features.