In heart catheterization (often called cardiac catheterization or cardiac cath) a very small, flexible, hollow tube (called a catheter) is placed in a blood vessel in the groin, arm or neck. Then it is advanced under x-ray guidance (fluoroscopy) into the aorta and towards the heart.
The catheter can be guided into the coronary arteries which supplies blood to the heart muscle. Contrast dye can be injected in the arteries to check for blockages/blood flow. This is called coronary angiography.