Streams of water. It's in the very first verse.
Read the whole Psalm to see the point the writer is making.
The imagery of the panting deer caught the imagination of song writers and it was turned into a popular praise and worship song that is used in many churches. You can see Salvador performing it on YouTube here.
The Psalms were all originally meant to be sung. That collection of writings was the hymnal for Israel.
The above photo is with a short devotion you can read here.
