iOS: Your phone is a very capable voice recorder, but you might miss an important moment in the few seconds it takes to find and open an audio recording app. Heard attempts to solve this problem by recording in the background, and saving a buffer of audio at all times.
Once you launch the app and start recording, it will keep working in the background while you go about your day. If you hear something you want to save, just hop back to the app and save the audio buffer locally, or send it via email or Facebook. Battery life is always a concern for anything running in the background, so this might be an app you use at your desk while plugged in, or only in situations where you think you might need it.
I'll be the first to admit, it sounds a little bit creepy, but Heard isn't uploading your audio to any third-party servers unless you explicitly share it, so the only thing you need to worry about is making sure the people around you know that you're recording. One potential use for Heard is in the office; if someone comes by and dictates instructions to you, Heard can save you from awkwardly fumbling around for a notepad or recording app.
Heard is a free app which saves a 12 second buffer in the background. A $1.99 in-app purchase will unlock 30 second, one minute, and five minute buffers as well.