Phew, that’s some existential-level question 
Adapter - An adapter is a pre-built plugin to connect to a target system, to access its data, execute certain remote operations and more. webMethods has a suite of adapters ranging from JDBC to Kafka (link).
Real-life Example - The concept is similar to a power adapter to your electrical appliances. Your appliance needs electricity to work, but you need an adapter to connect your appliance to the socket. Different appliances may have a different adapter, depending on the type of pins, power socket type, power output of the socket, country/region, etc.
JDBC Adapter - Let’s take the JDBC adapter for example. It is used to establish connectivity to a database that supports connectivity via Java (i.e., Java drivers). JDBC is an abbreviation for Java Database Connectivity (link).
Databases Supported via JDBC - Here, your database can be Oracle, Microsoft SQL Server, etc. This document provides you the list of databases that the webMethods Adapter for JDBC supports, i.e., the databases that you connect your webMethods Integration Server instance to, via this adapter. (link).
JDBC Adapter Documentation - Go through this documentation for the JDBC adapter and it will give you an overview (link).
What to do if your database is not in the list of supported databases?
In the rare occasion that this happens, you can use the Adapter Development Kit to develop your own adapter (link). This is an advanced topic, so I won’t go into the details.
Documentation for ALL Software AG products is available here (link).
P.S - You may come across the term CloudStreams Connector, which is not to be mixed up with Adapters (although its function may sound similar).
KM
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