Overleaf is a collaborative cloud-based LaTeX editor used for writing, editing and publishing scientific documents. It partners with a wide range of scientific publishers to provide official journal LaTeX templates, and direct submission links.
LaTeX differs from word processors such as MS Word because it is not handled using a WYSIWYG environment. Instead, the environment is much more akin to coding, where you will compile a given TeX document into a PDF. This makes it much easier to define macros, ensure that templates are followed, and to swap out images, all things that are invaluable in the context of scientific writing.