What is Tool
Developers call almost anything they work with a tool. Some tools are apps they type in, and some are code they build with.
In plain terms
The word covers two different things. React is code that becomes part of the product a developer builds. VS Code is an app the developer types in, and no part of it ships in the product. Both get called tools in conversation. When a tool name appears on a resume, the useful first question is which of the two kinds it is.
What it tells you about a developer
Tools that ship in the product, like frameworks and libraries, describe what the developer has built. Tools they work in, like editors and terminals, describe how they like to work. The first kind carries most of the hiring signal.