ASO is short for App Store Optimization. It is some kind of process of improving the mobile apps' visibility in an App Store. Just like Search Engine Optimization(SEO) is for websites, app store optimization is for mobile apps.
Generally, ASO is the effective process of optimizing mobile apps to rank higher in an app store's search results. The higher your app ranks in an app store's search results, the more visible it is to potential customers. That increased visibility tends to translate into more traffic to your app's page in the app store.