What does "flattening" an image in Photoshop do?

Flattening an image in Photoshop combines all visible layers into a single background layer. This process simplifies the image structure, making it easier to manage and work with, especially when preparing the file for final output. By combining layers, it also reduces the file size, which can be beneficial for saving and sharing.

Once the layers are flattened, any adjustments made to individual layers—such as opacity changes or blending modes—become a permanent part of the image. This is an essential step before saving images in formats like JPEG, which do not support layers, ensuring that the final appearance of the image is preserved.