From ifconfig(8). Please read both paragraphs.
Code:
nwflag flag
Set specified flag. The flag name can be either `hidenwid' or
`nobridge'. The `hidenwid' flag will hide the network ID (ESSID)
in beacon frames when operating in Host AP mode. It will also
prevent responses to probe requests with an unspecified network
ID. The `nobridge' flag will disable the direct bridging of
frames between associated nodes when operating in Host AP mode.
Setting this flag will block and filter direct inter-station
communications.
Note that the `hidenwid' and `nobridge' options do not provide
any security. The hidden network ID will be sent in clear text
by associating stations and can be easily discovered with tools
like tcpdump(8) and hostapd(8).