All modern application suites which have "rivals", as it were, can open each other's documents. However, each of these are of different "formats", which means the way the data they contain is arranged and categorised. So, e.g. Microsoft Word's default file format is .docx, but it can save its documents in a couple of dozen other formats so that they can be shared with other people who don't use Microsoft Word.
Similarly, LibreOffice and OpenOffice have their own default file formats in which documents will be saved unless you specify a different one. As Microsoft Office remains by far the most common suite of apps, it can make sense to configure LibreOffice / OpenOffice to use the Microsoft document formats by default. They, you pretty much don't have to worry about it.