From the Super Mario Wiki, the Mario encyclopedia
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L Blocks are a type of block exclusive to the Mario & Luigi series. They are very similar to the ? Blocks, though only Luigi or Baby Luigi can activate them. M Blocks are similar to these blocks, however, M Blocks can only be activated by Mario or Baby Mario. In Mario & Luigi: Superstar Saga, they are often used to solve puzzles. In Mario & Luigi: Partners in Time, they can be found scattered through different places. In Mario & Luigi: Bowser's Inside Story, they are mainly used for minigames to earn Attack Pieces. They may also contain items for the Mario Bros. to collect.
A similar block known as "Cap Block L", along with Cap Blocks M and W, were originally going to appear in Super Mario 64 DS and contained Luigi's Caps, but they were removed. The blocks can still be found in a test level.