Before the compiler can understand your program's meaning, it first breaks the source text into tokens — the smallest units that still carry meaning on their own. Understanding tokens explains why certain typos produce confusing errors, and why some words simply cannot be used as variable names.

A line like int total = a + b; is tokenized as: the keyword int, the identifier total, the operator =, the identifier a, the operator +, the identifier b, and the punctuator ; — seven tokens in total, with whitespace between them discarded once it has served its purpose of separating tokens.

Keywords are reserved words with a fixed meaning built into the language. The standard C keyword set (C11) is deliberately small:

An identifier is a name you choose for a variable, function, type, or label. Identifiers may contain letters, digits, and underscores, but cannot start with a digit, and are case-sensitive (total and Total are different identifiers).