Nintendo has released the Nintendo Switch Online App for iOS and Android, which will be used to manage certain online aspects of the Switch such as voice chat and matchmaking (since the Switch by itself lacks any built-in features for those). The app can be downloaded via Google Play Store and iTunes.
Splatoon 2 (which launches this Friday) will be the first game to make use of the app via SplatNet2, a Splatoon 2 exclusive service that helps keep track of your battles, ranking, stage schedules,, invitations, etc. The online is free for the rest of 2017, but will cost $20 annually from next year.