Meaning of I O requests on Amazon RDS

Question

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 now the billing is:

EC2:

2,000,000 I/O = free tier

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

**3,118,431 I/O TOTAL 

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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 

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I 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 for me?