Here a decent article on Meraki about coverage pattern and what they mean.
https://documentation.meraki.com/MR/WiFi_Basics_and_Best_Practices/Understanding_Wireless_Performanc...
Basically, the horizontal pattern is how the radio signal propagates on a horizontal plane if the access point or antenna, in this case, is mounted as expected. Vertical is for the vertical plane.
In the diagram, the horizontal plane shows it is directional in its facing but has a wide-angle, with a little going to the rear. It does extend to the left and right but not as strong as directly ahead. The vertical plane shows that it extends deep but not as wide so the frequency does not extend up or down as much. This is very desirable for point to point type connections which this antenna is designed for.
Indoor access points often look doughnut-shaped which is good for omni-directional coverage.
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CCNP Certified and Meraki Operator