Private key for authenticating to GitHub with, it must be in PKCS#8 format, GitHub will give it to you in PKCS#1.
You can convert it with openssl pkcs8 -topk8 -inform PEM -outform PEM -in current-key.pem -out new-key.pem -nocrypt
openssl pkcs8 -topk8 -inform PEM -outform PEM -in current-key.pem -out new-key.pem -nocrypt