LibreOffice 5.3 is a major update to this distant fork of OpenOffice.org. LibreOffice 5.3.0 is planned to be officially released in late January or early February while this week's alpha one is just the first step of the process. The hard feature freeze on 5.3 is at the end of November followed by a series of betas and release candidates. Those interested in more details on the release schedule can see this Wiki page.
Among the features coming to LibreOffice 5.3 are many user-interface improvements, new spreadsheet functions, some new templates for Impress, a better OpenXML filter, PDFs can now be inserted as an image into documents, and a range of other work. LibreOffice 5.3 should also offer faster rendering performance. Also, don't forget there still is a whole month to go until the hard feature freeze for 5.3 so more work may still come about.
More of the tentative LibreOffice 5.3 features/changes can be found via the in-progress release notes.