This is the simplest technique, one that you can learn in the app and practice anywhere you are. It is based on the fact that increasing chest pressure by taking very slow and deep breaths, and then reducing by slowly breathing out, triggers a reflex. This reflex slows down your pulse and gives you a relaxed feeling in your body. As body and mind are connected this then results in relaxation in your mind. Give it a go for 3, 5, 7 or 10 minutes!
Deep slow breathing is an essential technique incorporated in many relaxation exercises. It has been extensively examined in the literature in different setting. A good review of the evidence behind it and its uses can be found in General Principles and Empirically Supported Techniques of Cognitive Behavior Therapy, Chapter 14 by Hazlett-Stevens and Craske. You can read a preview of the chapter here.