I am going to assume that you didn't read wikipedia for what Ping is first. And define Ping as per my own basic understanding.
So, basically, Ping is a check by a PC to see if a website is responding.
If you Ping google.com for example, PC would send a small packet to Google.cm, and checks how much time does it take for the server to respond, or if it responds at all.
Code:
Pinging www.google.com [74.125.236.84] with 32 bytes of data:
Reply from 74.125.236.84: bytes=32 time=18 ms TTL=250
Reply from 74.125.236.84: bytes=32 time=21 ms TTL=250
Reply from 74.125.236.84: bytes=32 time=20 ms TTL=250
Reply from 74.125.236.84: bytes=32 time=33 ms TTL=250
Ping statistics for 209.85.135.105:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 18ms, Maximum = 33ms, Average = 23ms
Good Ping time and bad time are subjective. But generally Pings less than 100 ms are very good. Ping doesn't affect Internet speed. But it usually matters in online multiplayer FPS gaming, where you move fast, and want to shoot other player before they shoot you.