When you issue a "show interface <interface>" it will display the statistics for the interface selected:
GigabitEthernet1/0 is up, line protocol is up
Hardware is GigMac 3 Port GigabitEthernet, address is 000b.bf2e.9100 (bia 000b.bf2e.9100)
Internet address is VVV.XXX.YYY.ZZZ
MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec, rely 255/255, load 13/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex mode, link type is force-up, media type is SX
output flow-control is unsupported, input flow-control is off
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:00, output 00:00:00, output hang never
Last clearing of "show interface" counters 14w2d
Queueing strategy: random early detection (WRED)
Output queue 0/40, 0 drops; input queue 0/75, 0 drops
5 minute input rate 24283000 bits/sec, 9653 packets/sec
5 minute output rate 53901000 bits/sec, 10016 packets/sec
42635042229 packets input, 15110789127436 bytes, 0 no buffer
Received 155 broadcasts, 0 runts, 1123639 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 3586458 multicast, 0 pause input
41129255803 packets output, 26839029196142 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 babbles, 0 late collision, 0 deferred
1 lost carrier, 0 no carrier, 0 pause output
0 output buffer failures, 0 output buffers swapped out
As you can see, it has several counters which increment when the corresponding traffic matches. When you are troubleshooting, it is useful to have a reference value for this counters. That's when the "clear counters <interface>" command comes handy, because it resets all counters so it is easier for you to determine which counters are incrementing.