Shutter speed determines how long the sensor reads the image during the frame. So if you are shooting at 30 fps and shutter speed of 1/90 you'd have open shutter for 1/90 of a sec, and closed shutter for 2/90 of a sec. This affects motion blur in the frame, just like it would on a still. For the most "filmic look" you should use 1/50 on the 5dmkii at 24 fps. A film camera would normally shoot at 24 fps with 1/48 shutter speed. This is also known as 180 degree shutter angle, where the shutter is open for the same duration, each frame, as it is closed.