NodeJs is an open source runtime environment. Runtime simply means something that is executable in it's own environment. Nodejs was built on google's chrome v8 javascript engine.
Node is not a convention based platform where everything is thought through such as the database, sessions, authentication, etc. For example PHP, Ruby on Rails etc. NodeJs has no idea or does not know which database you are going to hook on to.
Node is not recommmended for high CPU intensive operations like reading the file system asynchronously, or machine learning or large arithemetic. Node is good when it comes to concurrency.