A raw, uncompressed video file is enormous, often gigabytes per minute, because it stores the full color value of every single pixel in every single frame with no shortcuts. Compression exists to make that manageable, and it works by exploiting a simple fact: most of a video frame barely changes from the frame before it. A codec identifies what's redundant, throws it away or approximates it, and only stores what actually changed, which is why understanding the three main levers, codec, bitrate, and resolution, explains almost everything about why a compressed video looks the way it does.
What a codec actually does
A codec (short for coder-decoder) is the algorithm that decides how to compress and decompress video. Most codecs use two tricks: intra-frame compression, which compresses each frame somewhat like a JPEG image by grouping similar colors, and inter-frame compression, which stores only the differences between consecutive frames instead of redrawing the whole picture each time. Newer codecs like H.265 (HEVC) do this more efficiently than older ones like H.264, producing a smaller file at the same visual quality, though they need more processing power to encode and aren't supported quite as universally. The Video Format Converter lets you switch between codecs and containers without re-shooting anything.
Bitrate: the setting that matters most
Bitrate is the amount of data allocated per second of video, usually measured in Mbps. It's the single biggest lever on both file size and visible quality: a low bitrate forces the codec to approximate more aggressively, which shows up as blocky artifacts, smearing, and loss of fine detail, especially in fast motion or busy scenes. Two videos at the exact same resolution can look completely different if their bitrates differ, which is why bitrate, not resolution, is usually the first thing worth adjusting when a compressed video looks noticeably worse than the original. The Video Bitrate & Resolution Changer lets you set a specific target bitrate directly rather than relying on a vague "quality" slider.
Resolution: fewer pixels, less data, but a real trade-off
Resolution determines how many pixels make up each frame. Halving a video's width and height (say, from 1920×1080 to 960×540) cuts the pixel count to roughly a quarter, which dramatically reduces file size, but it also permanently discards detail that can't be recovered later. Resolution reduction is most worth using when the final viewing size is genuinely smaller, like a video destined for a phone screen or an embedded thumbnail, since shrinking resolution for content that will be viewed full-screen produces a visibly softer result than lowering bitrate at the original resolution instead.