Redis configuration question

Hi this may seem as a pretty stupid question, but when configuring Redis is says the following:

the object cache drop-in will connect to Redis over TCP at 127.0.0.1:6379 and select database 0

Now what exacly is database 0

I have 5 databases the output below is an example.

Now if my wordpress database is the one called lemon as in the example below how can the setting “database 0” work? Would it not be another number rather than 0 since there are several databases. Or is the database parameter something else entirely.

MariaDB [(none)]> show databases;
+——————–+
| Database |
+——————–+
| information_schema |
| mysql |
| lemon |
| mymothershamster |
| performance_schema |
| spiderman |
| unameit |
+——————–+

Thank you.