Block rating was a number typically appearing on items that improved your chance to block an attack. Some abilities, enchantments, items and spells could contribute bonus block rating as well (the bonus spell usually has the name "Improved Blocking" or "Increased Block X").
At level 80, players required 16.4 block rating to gain 1% block chance. Block chance could also be increased through defense rating, of which 122.4 rating is needed for 24.88 defense to give 1% block chance.
Block rating is not to be confused with "block value" which increases the amount blocked each time a block occurs, rather than the chance for a block to occur.