What Is Video Compression? Bitrate, Codecs, and File Size
Uncompressed video is enormous — genuinely too large to store or transmit practically for almost any everyday use — so video compression isn't an optional nice-to-have the way it sometimes is for images; it's the reason digital video is practical to use at all.
Where video compression finds its savings
Video compression exploits two main kinds of redundancy: within a single frame (similar to image compression, finding redundancy in the visual detail of one frame), and — more significantly — between frames, since consecutive video frames are usually very similar to each other. Rather than storing every frame in full detail, video compression can store a full frame occasionally and then, for the frames in between, store only what actually changed — which is dramatically more efficient for typical video content where most of the frame stays relatively static from one moment to the next.
Bitrate: the setting that matters most
Bitrate describes how much data is used per second of video, and it's the primary lever controlling the balance between file size and visual quality. A higher bitrate allocates more data to represent each second of footage, generally producing better quality at a larger file size; a lower bitrate compresses more aggressively, shrinking the file at the cost of more visible compression artifacts, particularly during fast motion or highly detailed scenes.
Why resolution and bitrate are different settings
Resolution describes how much detail is captured within a single frame (1920×1080, for instance); bitrate describes how much data is spent encoding that detail over time. It's entirely possible to have a high-resolution video at a low bitrate (looking soft and artifact-heavy despite sharp source footage) or a lower-resolution video at a generous bitrate (looking clean and detailed within its smaller frame size). The two settings solve related but genuinely different problems.
Why re-compressing already-compressed video hurts quality
Since video compression is lossy, each time a video is compressed, some quality is discarded — compressing an already-compressed video a second time compounds that loss rather than starting fresh, which is why it's generally better to compress once, directly from the highest-quality source available, rather than repeatedly re-compressing an already-compressed file.
Compress Video gives direct control over bitrate-based compression in your browser, with an estimate of the resulting file size. Convert Video is useful when a different codec, rather than just a different bitrate, is the better route to a smaller or more compatible file.