The sentence you wrote is almost correct and very clear! Here's a slightly improved version:
"Saguaros are covered in wrinkles and ribs. When it rains, these ribs stretch out to hold all that water inside."
It's great as it is! The words "wrinkles" and "ribs" describe the cactus well, and your explanation about how the ribs stretch when it rains is easy to understand. If you want a tiny improvement, you can add a little more detail like this:
"Saguaros are covered in wrinkles and ribs. When it rains, their ribs stretch out to hold all the water inside."
This uses "their" instead of "these" but both are correct. Good job!