Apple is trying to find the best match for the users search query (keyword).

Things you can do in your app listing

• If there are not many good results for the keyword, then Apple will prioritize the app title matching closest to the keyword
• Apple views the App Title as the biggest indication of what the app does. Specifically the left most words (beginning of the title) are biggest priority.
• Apple also considers the words in your app subtitle and keyword fields.
• The biggest sin in ASO: don’t repeat words in your title, subtitle and keyword fields.
• You can use the title, subtitle, keyword fields of other (certain) localizations of your app to specify even more keywords. Again, don’t repeat words.
• Apple doesn’t care about the app description.
• Apple considers the conversion ratio of your app for that keyword. If someone searches for that keyword and chooses your app to download, that’s a big signal that your app is relevant for that keyword.
• Optimizing for a keyword involves more than just the title/subtitle and keyword fields. To improve your conversion rate for a specific keyword, use your screenshots and app icon to let the user know that your app is relevant for his/her query (keyword).

Things you can do in your app

• Apple considers how well you retain your users. Do they keep coming back?
• App crashes (also tracked by Apple) negatively impacts your rankings.
• Apple considers how many reviews the app got in the last 30 days as a ranking factor. Reviews can be good or bad. But bad overall rating may turn users off from downloading it.
• Once you rank well for your target keyword, you can target and optimize for another keyword. You will still rank for the first keyword because Apple has gathered enough data that your app is relevant for the first keyword. You’ll keep your rankings for the first keyword while you’re optimizing for the second one.
• Apple likes frequently updated apps. An app that hasn’t been updated in a long time is a negative signal.

Other things to note

• Apple gives your newly launched app an initial boost so that it can gather data about your app.
• Gaming the system/blackhat techniques (buying reviews, paying for downloads etc) are not worth the risk because Apple will ban you if they find out. You won’t be able to make new accounts.