List of credentials to be used by the sshagent step. The Pipeline Syntax Snippet Generator will guide the user to select a single credential from the list of available, like this:
sshagent(credentials: ['my-credential-id']) {
    // some block
}
The step also accepts a list of credentials, like this:
sshagent(credentials: ['my-credential-id', 'my-alternate-credential', 'another-credential']) {
    // some block
}