Video resolution is the number of pixels in each dimension of a video frame. Standard resolutions: 480p (854×480, SD), 720p (1280×720, HD), 1080p (1920×1080, Full HD), 1440p (2560×1440, 2K/QHD), 2160p (3840×2160, 4K UHD), and 4320p (7680×4320, 8K). The "p" stands for progressive scan (all lines scanned each frame), as opposed to "i" for interlaced.
Resolution has a direct impact on file size — 4K contains 4x more pixels than 1080p, and 4x more data to encode. However, the relationship isn't perfectly linear because higher resolution videos benefit from temporal compression across all those pixels. For web video, 1080p is the sweet spot: excellent quality visible on most screens, manageable file sizes. 4K only provides visible benefit on screens larger than 27 inches or when cropping is needed.
Platform recommendations: YouTube supports up to 8K; 1080p is standard for most creators. Instagram caps at 1080p. TikTok displays at up to 1080p. WhatsApp compresses to ~720p. For phone recording, 1080p at 30fps provides the best size/quality balance for most purposes.