What is performance testing?

Performance testing is used to ensure a good user experience which is fast and error free under any user load by creating the user load and measure the user experience.

JMeter

The Apache JMeter™ application is open source software, a 100% pure Java application designed to load test functional behavior and measure performance. It was originally designed for testing Web Applications but has since expanded to other test functions.

How does JMeter works?

JMeter sends a HTTP request
Server responds it(images,HTML etc)
JMeter parses response(optional)
Repeat