On iOS by default, an established VPN connection gets disconnected if there is no network use for a while, or sometimes when the device is in 'Sleep' mode.
The VPN then has to be manually reconnected for any further network use.
The Always-On / EverSecure feature in the ZenMate iOS App overwrites this default behaviour which forces the VPN connection to reconnect on demand, i.e. each time the device connects to the internet.
If the EverSecure feature is enabled and you "kill" the Zenmate app (swipe it from recently used apps), the VPN disconnects for a few seconds, before being reconnected in the background (please note that the VPN only reconnects if it was connected before you "killed" the Zenmate app).