Literally just adds trident shards. You need 3 to craft a trident, and each slain Elder Guardian drops 1.

Simply place a prismarine shard in the bottom-left and middle, with trident shards in the remaining 3 top-right crafting table slots. Note that you will unlock the recipe in your recipe book automatically upon collecting your first trident shard.

The trident shard texture used is from Additional Additions