You can import videos directly from your phones' video library so that you can edit those videos and share. Tap the Import button on the upper left of the app screen and then select you video to import.
Having trouble importing?
Make sure that your videos are saved in a compatible format.
On iOS, you can access your camera settings by visiting, Settings > Camera > Formats and choose Most Compatible.
The settings vary on Android devices, but they are typically in the camera app. The Screencast-O-Matic app cannot import videos that are saved in High Efficiency (HEIF or HEVC) mode. You'll want to choose JPEG/H.264 or Most Compatible.