Urodynamics is the investigation of functional disorders of the lower urinary tract, i.e. the bladder and the urethra. Symptoms are particularly unreliable in the study of the lower urinary tract, so the purpose of urodynamics is to confirm objectively the pathology that a person's symptoms would suggest.
So for example if a person complains of a high frequency of urination it might be because of detrusor overactivity. Urodynamics would involve the use of pressure transducers attached to catheters inside the bladder and rectum to verify such a pressure pattern.
Detrusor overactivity is characterised by the phasic pressure contractions seen in the image on the right. The red trace is the abdominal pressure usually recorded in the rectum, the blue trace is the actual bladder pressure (usually called intravesical) and the purple trace is the detrusor pressure - the part of the intravesical pressure that is entirely due to the detrusor muscle.
This is not a normal finding. The normal bladder should be low pressure during filling.