The really simple explanation is that Venus' orbital period is 225 days. Earth's is 365 days.
That ratio is 13:8 (well, more accurately 12.9777777777..:8) so you can see that your complete cycle is 13 orbits of Venus to 8 of Earth, but within that cycle our view of Venus travelling round the sun gives the pretty pattern as shown in your image.
This YouTube video is quite a nice visualisation - should make it easy to understand.
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