A Law Lord, officially called a Lord of Appeal in Ordinary, was a judge given a place in the British House of Lords. The law was changed in 2009, so no more have been made since then. Since then judges are made Justices of the Supreme Court of the United Kingdom.
Before 1876 the House of Lords was the highest court in the United Kingdom, but it did not actually have judges. After that the most senior judges were made members, and formed a committee to hear appeals. There were 3 to start with but the number was increased to 12.