What is the use of BaseColumns's _ID primary key in Android

It is a bit confusing.... If I implement BaseColumns for each of my tables it automatically create an autoincremented primary key for me called _ID but then Do I need to create my own primary keys for each of my tables as well or is it redundant or even not necessary?

In case I need my own primary key, let's say, _MyID, I guess the primary key will be formed by _ID and my own (_MyID), right? so in this case, it would be possible to insert the more than one register with the same _MyID.... as _ID is autoincremented automatically, that is:

_ID      _MyID  Other Fields.....
1         1000  ....
2         1000  ....
3         1000  ....

... and so on

so in this case, how to control that only one register can have the value 1000 for _MyID?

Also, I guess I can use _ID column to act as a foreign key with other tables, right?