What is “Signature Verification Failed” in Solana?

I’m trying to call a Solana Program, and when I run sendAndConfirmTransaction, it gives me Signature Verification Failed, and I’m not sure why.

const {sendAndConfirmTransaction, clusterApiUrl, Connection} = require("@solana/web3.js");

let signer = Keypair.generate();
let anotherKeypair = Keypair.generate();

let transaction = someInstruction(signer, anotherKeypair); 
let connection = new Connection(clusterApiUrl('testnet'));

sendAndConfirmTransaction(
  connection,
  transaction,
  [signer]
);