This error means your CA root certificate is out of date. If your CA root certificate, you won't be able to push or pull from GitHub repositories.

The error you receive may look like the following:

git push -u github.master
fatal: 'github.master' does not appear to be a git repository
fatal: The remote end hung up unexpectedly

git pull -u github
error: SSL certificate problem, verify that the CA cert is OK. Details:
error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed while accessing https://github.com/tqisjim/google-oauth.git/info/refs
fatal: HTTP request failed

A "CA" is shorthand for a "certificate authority," a third-party group responsible for handling secure connections around the web. They establish digital "certificates," which are a way of ensuring that there are valid connections between two machines (like your computer and GitHub.com). Without a certificate, the security risk between two machines is greater.

When you receive this error, it likely means that your CA is out-of-date and needs to be updated. Generally, updating your operating system also updates your CA, and solves the problem.