A weight (重り, omori) is a heavy object used for training by weighing down the user. Rock Lee wears a pair of ankle weights with the kanji, meaning "guts" (根性, konjō), written on them. During the Chūnin Exams, Might Guy told Lee to remove them while facing Gaara, which resulted in his speed being increased substantially. The weights are heavy enough to leave a small crater on the floor.
In Other Media
In the first three games from the Ultimate Ninja series, Lee's weight appeared as an item that can be thrown on opponents to temporarily reduce their speed. It is named Weight of Gut (根性のおもり, Konjō no Omori) in the first and second games and Weight of Determination in later releases. In the fourth game onward it was replaced by a tag with the same effect, the weight having minor or no appearances afterwards.
In Naruto the Movie: Guardians of the Crescent Moon Kingdom, Rock Lee wears heavy bars on his ankles. Besides their training purpose, those bars can also be combined into weapons like a pair of nunchakus, a bō staff, and a long chain. Rock Lee used them in his fight against Kongō.