A normal aspect of a wireless deployment is the expectation that clients will roam between access points. Under certain circumstances a client may not disassociate gracefully when roaming (client is out of range of the original AP, wireless card was reset, etc). This can result in a period of time where two or more APs believe they have the same client associated to them, leading to multiple access points replying to the client which causes connectivity problems. To help mitigate this scenario, Meraki APs use a "Magic ARP" to inform other APs that a client has associated.
When a client associates to an MR AP, that AP crafts an ARP frame spoofing the client's MAC with a 6.x.x.x IP address for uniquely identifying the AP sending the ARP. The spoofed client MAC will denote which client other APs should drop from their association table if they still show the client as associated. The magic ARP is sent as a broadcast by the AP both wirelessly and over the wire for the highest probability of reaching other MR APs in the network.