A 2-second continuous video play is when someone views a video for 2+ seconds continuously, usually with at least 50% of the video in view.
A 3-second video play is recorded when someone views a video for 3 seconds or views 97% of a video that is less than 3 seconds long.
An ad set is a Facebook ads grouping where settings like targeting, scheduling, optimization, and placement are determined.
Facebook has advertising specifications that apply to copy (character count) and creative (dimensions, aspect ratio, format) by placement.
An app activity custom audience is an audience used for ad targeting of people who launched your app or took specific actions while using it.