Apache JMeter is a Java open source tool.
If you want to run load or performance tests using Apache JMeter, you can use their Graphic User Interface (GUI) and you don’t need to know any programming language, follow the JMeter documentation and you are ready to go to design your API testing scenarios.
Depends a lot on your tests that you want to do with JMeter, at the moment there are a lot of Samplers and a lot of tasks come out of the box, or through other plugins that can be easily be installed with the JMeter Plugins Manager.
However, if you want to extend the functionality, you can use Groovy, which is the recommended scripting language for Apache JMeter.
Groovy is easy to use and flexible.
For assertions or samplers, you can use BeanShell programming scripts, but most of the tasks can be accomplished nowadays with builtin JMeter functionality.
Regular expression may come in handy also when creating your JMeter script assertions, so regex knowledge might be useful.
With JMeter Load Testing from LoadFocus you can run tests with thousands of concurrent users from multiple world regions in the cloud, all within minutes.