Search is how you find things you know are there. Discoverability is the measure of how well an application presents features to you that you didn't necessaril

Search is how you find things you know are there. Discoverability is the measure of how well an application presents features to you that you didn't necessarily know were there. A menu may be a list of features, when you open the menu you discover those features. A search box is a box. It shows no features. Search results might show features but those features might also not relate at all to the task you're doing, which means they're out of context. That's extremely poor discoverability. You can find all the features, you can't discover them.