Imports the public key from an RSA key pair that you created with a third-party tool. Compare this with CreateKeyPair, in which AWS creates the key pair and gives the keys to you (AWS keeps a copy of the public key). With ImportKeyPair, you create the key pair and give AWS just the public key. The private key is never transferred between you and AWS.
For more information about key pairs, see Key Pairs (http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-key-pairs.html) in the Amazon Elastic Compute Cloud User Guide.