Let's see an example:
# nmap -sS foo
Starting Nmap 6.40 ( http://nmap.org ) at 2014-06-06 15:16 BST
Nmap scan report for foo (XXX.XXX.XXX.XXX)
Host is up (0.0036s latency).
Not shown: 996 closed ports
PORT STATE SERVICE
22/tcp open ssh
5900/tcp open vnc
5901/tcp open vnc-1
6000/tcp open X11
MAC Address: B8:27:EB:XX:XX:XX (Raspberry Pi Foundation)
Nmap done: 1 IP address (1 host up) scanned in 0.26 seconds
What should one take from this result in easiest possible terms especially if someone is from application security background?
I'm not sure what you mean by "easiest possible terms", but from the nmap scan I showed you above one can see
- that the host is responding
to ICMP probes (text removed after comment, see below)
- that it has a bunch of open ports
- that it is likely to be a Raspberry Pi
At this point you might want to investigate these open ports, or try a
nmap -sU to check open UDP ports...
This question may sound stupid but I would prefer if people can connect all the dots from an adversary perspective.
If you meant to ask "what can an adversary do", the only limit is time, inspiration and creativity.
nmap is a first point of contact with a remote host; it comes with drawbacks and caveats as most port scan can be detected (however see this for a fascinating read).
In practical terms, once you see what's open, the first step might be to assess what's actually running there; check whether it's actually protected; see if you can gain more information e.g. on the version of the servers running; perhaps enumerate username (so you can test them on another machine), and so on.
A commenter below made me notice that:
If the target had been on a different link-layer segment (i.e. one or
more IP hops away), then ICMP would have been used (among other
probes), and no MAC address would be shown. The only way you can get a
MAC address and use ICMP for discovery is to add --disable-arp-ping or
--send-ip. Even explicitly requesting ICMP Echo with -PE will use ARP for discovery otherwise
The current revision of the answer has been corrected.