I have two cAP's. When I compare both in Interface <wlan2>, the details at the bottom of the window are different.

One cAP shows "enabled" , "running" , "slave" , "running ap"
The other cAP shows "enabled" , "" , "slave" , "running ap"

But during writing this they both now show "enabled" , "running" , "slave" , "running ap".

"enabled" & "running ap" seem pretty obvious to me.

Can someone explain "running" & "slave"?

"slave" is mostly when your WLAN is a port on the bridge. The bridge is the master interface. IP settings must be set on the master.
A virtual WLAN will be slave to the physical WLAN.
The WLAN interface stops "running" when there is no client connected. It will be "running" when a client device is connected. (R in the table view)