ifconfig is a utilty can used for getting information about network interfaces and stack, chagning them or adding virtual interfaces.
running ifconfig without any options will show usually something like this:
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 32768
index 3 priority 0 llprio 3
groups: lo
inet6 ::1 prefixlen 128
inet6 fe80::1%25lo0 prefixlen 64 scopeid 0x3
inet 127.0.0.1 netmask 0xff000000
em0: flags=8b43<UP,BROADCAST,RUNNING,PROMISC,ALLMULTI,SIMPLEX,MULTICAST> mtu 1500
lladdr aa:bb:cc:dd:ee:ff
index 1 priority 0 llprio 3
groups: egress
media: Ethernet autoselect
status: active
inet 12.34.56.78 netmask 0xffffff00 broadcast 12.34.56.78
inet6 2002::ffff:ccff:fedd:ee06%25vio0 prefixlen 64 scopeid 0x1
inet6 2002:ffff:2:1040:: prefixlen 48
enc0: flags=0<>
index 2 priority 0 llprio 3
groups: enc
status: active
pflog0: flags=141<UP,RUNNING,PROMISC> mtu 33136
index 4 priority 0 llprio 3
groups: pflo
besides em0, all other interfaces mentioned (pflog0, enc0, lo0) are virtual and they will exist on all openbsd installations most of times.
em0: flags=8b43<UP,BROADCAST,RUNNING,PROMISC,ALLMULTI,SIMPLEX,MULTICAST> mtu 1500
it shows em0's mtu (maximum size of a packet) and flags set on it. some can changed using software, some are hardware-dependent and can't without changing hardware.
lladdr aa:bb:cc:dd:ee:ff
this refer about MAC adderss used by this interface, which is aa:bb:cc:dd:ee:ff
inet 12.34.56.78 netmask 0xffffff00 broadcast 12.34.56.78
this is referring to ip address, netmask and broadcast address
ip address can be local, if you are using NAT
inet6 2002:ffff:2:1040:: prefixlen 48
inet6 is like inet but for ipv6.