A lumbar puncture (also known as a spinal tap) is a procedure used to take a sample of cerebrospinal fluid (CSF) from the spinal canal so that it can be tested. The spinal canal is the passageway through the back bones (vertebrae) which contains the spinal cord.
A hollow needle is inserted into the lower part of the spinal canal to draw out the CSF (see How it is performed). Usually only a few millilitres are taken. The procedure is very safe and takes place routinely without complications.
Definition from NHS Choices: lumbar puncture