As Tom states, that's interlacing.
Interlacing was invented when the TV was born, to carry 1/5 the frame with each Hz of the electricity rating. In the US, that's why you use 30 fps (or 29.nnn) in the US and we use 25 fps in europe as the electricity is 60Hz/50Hz.
Interlaced video should ONLY (I repeat ONLY) be used for analog TV signal transports.
Each and every digital TV set today, including all the computer screens, is Progressive, updating the full frame at each screen refresh. And most TV's even refresh much quicker, newest TV's refresh a 800Hz..! (giving then problems with the newest LTE spectums..)
If your source media in interlaced, ask why! And stop shooting interlaced.. ;-)
If you have interlaced video, make sure you apply the DeInterlace setting when you import and transcode it.