When you upload a video to youtube, it will initially be processed in low resolution. This process helps you complete the upload process faster. When the upload flow is complete, your video will be available to stream in low resolution, on a wide variety of devices.
Higher resolutions, such as 4K or 1080p, can take more time to process. This process takes a few hours! During the processing time, your video will appear in low-res. Once the high-resolution processing is finished, your video will be available in high-res!