To castrate a male animal is to surgically remove its testicles. A veterinarian castrates a male dog to ensure that it can't mate with a female and produce unwanted puppies.
People often choose to castrate pets or farm animals to keep them from reproducing. Castrate is one of several words used to refer to this process; most pet owners prefer terms such as neuter or fix. Castrate comes from a Latin root, castrare, which means not just "to castrate," but also "to prune." The word has also come to mean "to take away strength."