Meaning of I/O requests on Amazon RDS

Question

I really tryed but I can not find this information over internet. I'm using the AWS Free Usage Tier, I have one EC2 (t1.micro) instance with Windows Server 2008 and one RDS (t1.micro) instance. After one month of usage with a really small website (about 30 visits per day), it has just one contact form, about 6 inserts on the database per day, now the billings:

EC2:

2,000,000 I/O = free tier

1,118,431 I/O = $0.16 exceeded

**3,118,431 I/O TOTAL - I have no ideia why so many I/O

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RDS:

10,000,000 I/O = free tier

123,715,372 I/O = $17.32 exceeded

**133,715,372 IO TOTAL - I have no ideia why so many I/O

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I really want to know what one I/O means, I thought that one I/O = one request, but now I think that is not it. Somebody can clarify me?

Answer 1

Finally, after more than one month I figure out where was the problem:

I found a thread on AWS Forums about MS SQL dev DB continuously at 100%, an Amazon employee talked about "canceled queries that must is still running". Their answer:

"then your only option here is to try to Reboot your RDS 
 instance which should stop the query from being executed"

So, I did. Now my I/O requests was reduced on 90%.