Every program stores and manipulates data, and in Java how you store it is governed by a strict, static type system: every variable has a declared type fixed at compile time, and the compiler enforces that you only use it in type-appropriate ways. This is a deliberate design choice that distinguishes Java from dynamically-typed languages, and it is the source of much of Java's reliability, since a huge class of errors is caught when you compile rather than when your program is running in front of users.
Java draws a fundamental line between primitive types, the eight built-in value types like int and double that hold raw values directly, and reference types, objects accessed through references. Operators, arithmetic, relational, logical, and more, are the verbs that act on this data, and understanding how each behaves is essential to writing expressions that compute what you intend.
Understanding variables, types, and operators precisely matters because they are the vocabulary of every line you will write. Java's static typing means getting them right is not optional politeness but a contract the compiler enforces, so a clear mental model of what each type holds, how much space it uses, and how operators behave on it prevents subtle bugs that would otherwise surface only at runtime.