Class 10 indicates that the card provides write speed of at least 10MB / s.
UHS-I U1 means that the card is fast enough for Full HD video recording at 1080p (i.e. > 10MB/s constant writing).
UHS-I U3 means that the card is fast enough for shooting videos in 4K Ultra HD (i.e. > 30MB / s constant writing) with devices that support the UHS-I standard.