Multer is a node.js middleware for handling multipart/form-data, which is primarily used for uploading files. It's written on top of busboy for maximum efficiency. Multer adds a body object and a file or files object to the request object. The body object contains the values of the text fields of the form, and the file or files object contains the uploaded files, depending on the configuration. Multer simplifies file uploads significantly in Express applications.