Due to restrictive mapping laws in Korea, foreign navigation apps are quite limited. This makes South Korea one of the very few countries where Google Maps does not work well, and is not used by locals. Recently, Google Maps in Korea has been improving, but for staying more than a couple of days, using Korean navigation apps will be much better.
The main navigation apps in Korea are Naver Maps and Kakao Maps. Both apps are fairly easy to use, and can mostly be used in English.
Which one should you use? This somewhat comes down to personal preference, but here is the context:
If staying in Korea for a while, it would be reasonable to set up both and, for example, use Naver for navigation and opening times, and Kakao for finding high-rated places.
Unfortunately, much of the content on both Naver Maps and Kakao Maps is still only in Korean, especially surrounding restaurant details and detailed transit information. Additionally, both apps are often very unreliable when it comes to searching English addresses, or English search terms such as "Grocery Store". The companies have been pushed to improve in the past, but have cited lack of foreign users, and difficulty of quality control as reasons for lagging behind. As crazy as it seems, a decent strategy is to take screenshots and run them through Google Translate.