This project is a simple example of a Java application accessing a SQLite database. There is ABSOLUTELY NO WAY you should run this in a real project, as it is really, really insecure. Just don't do it. Learn from it, but that's all.
The later versions (from 2.0.0) also showcase how to expose the database as a REST API using the Javalin framework. Thus, Java 11 or later is required.
You can easily build the code directly from within your IDE of choice (I'm personally rather fond of Eclipse), using its build features (in Eclipse, right click the project and select Run As -> Maven build, type package into the Goals field, then Run). If you prefer doing stuff more old school, you can use the command line to build the project as well. To do so, you need to have Maven installed on your machine. Navigate to your project location. Then, simply type
mvn package
in your terminal. Your generated JAR files can be found as ./target/java-backend-2.0.0.jar and ./target/java-backend-2.0.0-jar-with-dependencies.jar.
The easiest way of running the code is directly from within your IDE of choice, simply by hitting the Run button (in Eclipse, that is). If you prefer running your code from the command line, navigate to the project location and type
java -jar target/java-backend-2.0.0-jar-with-dependencies.jar
Don't forget to kill the process when done (using the stop button in the Eclipse console, or by hitting ctrl-c in the terminal).