how to protect a table with a password
Hello,
There is no short answer for such a general question as this. It is not clear as to what you are trying to accomplish.
First understand the two major pieces involved here - Base and database. Base is just a front end to a database. Using Base as delivered with LibreOffice you will be using the HSQLDB v1.8 embedded database. You have the option of using a variety of databases with Base such as PostgreSQL, MySQL, Firebird, Oracle and more.
The only Base passwords I can recall involve password protecting macros. Everything else in base is just screens and report creation which is generated by connecting to a database which is not actually part of Base.
A database, depending upon which one is being used, can have multiple schema which contain items such as tables, queries, triggers and more. Databases have users defined which can be given access to specific items in different ways. For example, UserA may be able to read & write to TableA in a schema but UserB can only read TableA. Another user may have no access at all to TableA. This is just a simple example. There are many other aspects to capabilities such as modifying or creating tables. In controlling this, each user has their own username & password to sign in with.
The best way to approach setting something like this up is to refer to the documentation of the version of database you are using. Each version of a database can be different as capabilities change.
Now, if you are dealing with the default installation from LO you will not be able to do much of this without a great deal of problems. For one, this is a very old version of database and is currently being replaced with Firebird embedded v3.0. Embedded databases are not intended for multi-user access. For this an external DB is preferred.