Base is a database tool, and an integrated component of LibreOffice. You can start it from Finder - Applications, from the LibreOffice core app, or from menu File - New … in any of the user apps.
Databases are structured collections of data. Typical uses include statistics and accounting, but there are many other uses. (E.g.: The engine inside this forum relies heavily on a database.)
As far as I know, you can’t open an Access database file using Base, but you can connect to it. That distinction may be elusive.
When you connect to a database, you can access data manipulation objects - tables, relationships and queries mainly - to extract or modify data.
When you connect to a database (be it an Access file, a spreadsheet, an external database or something else), a Base odb file is created to store connection info, queries and other items which Base can’t save into the connected database.
When you open a database file, you also have access to “presentation items”, like forms for data entry, reports and other items.