A listener basically captures results received from the server. We can capture individual results to consolidated results with the help of different listeners. The listener can be added at the test plan level, thread group level or even under individual requests. When added under the test plan level, results will be generated for all the requests consolidated. Most of the listeners perform several roles in addition to "listening" to the test results. They also provide means to view, save, and read saved test results. They collect the results of your runs. You can select among graphical, tabular, single request views. The results can also be saved in a named file.

Following are some important listeners in JMeter:
Aggregate Graph
Aggregate Report
Assertion Results
Graph Results
Monitor Results
Simple Data Writer
View Results in Table
View Results Tree

Note: Listeners consume a lot of memory if there are a lot of samples. So in practice this is not recommended to use lots of Listeners while execution.