monaco-editor-core
This npm module is a building block for the monaco-editor npm module and unless you are doing something special (e.g. authoring a monaco editor language that can be shipped and consumed independently), it is best to consume the monaco-editor module that contains this module and adds languages supports.
The Monaco Editor is the code editor that powers VS Code, a good page describing the code editor's features is here.
This npm module contains the core editor functionality, as it comes from the vscode repository.