Need a simple explaination for this. what is a lumbar puncture?

Retrieval of fluid: A lumbar puncture is a diagnostic procedure in which a sample of the fluid that bathes the brain and spinal cord is retrieved for analysis. This is useful in the diagnosis of multiple diseases, including multiple sclerosis, meningitis, encephalitis, and some autoimmune diseases. In some diseases with elevated fluid pressure, such as pseudotumor cerebri, a lumbar puncture can also be therapeutic.