Spiritfarer has a variety of resources you need to collect to create unique items. You can use them in recipes to make further modifications to your ship or to make meals and items for the spirits you take to your boat. A more complicated and extraordinary resource you want to try to find are comet rocks. You can only collect them from a particular location. They work a bit like the jellyfish you first collected when you started.
Before you can find them, there are two things you need to do. You first need to meet Giovanni and then add the rock destroyer upgrade to your ship. When you have both of these available, Giovanni pulls you aside to give you a small mini-event to teach you about the Comet Shower. This event provides you with comet rocks. Unlike the jellyfish event, the comets fall onto your ship, and you move around your boat to pick them up.
After that, Comet Shower events show up randomly throughout the game, but only in the southeast region of the map, so you need the rock destroyer. It gives you access to this area, enabling you to locate a Comet Shower.
To find Comet Showers on your own, be on the lookout for distinctly blue and green colored sea regions where you can see falling stars. The red and orange falling stars should be a distinct difference to the eerie glowing blue. These events will be similar to the one Giovanni showed you.