Your computer has a built-in list of signatures that it trusts and expects to see on the certificates, and one of those signatures was eMudhra’s. These signatures are basically just a proof that you know a secret number without revealing the number itself. If you know the secret number, you can sign the certificate.
There are an unimaginable number of secret numbers. Imagine each atom in the Great Pyramid of Giza is a secret number. And then imagine you have as many Great Pyramids of Giza as there are atoms in the universe. And then double it.
Now imagine you choose any of the numbers represented by all those atoms to sign the certificate with, and you chose 4.
It’s not technically wrong: 4 is indeed a number. But if anyone wants to guess your number, and they start at 0, it won’t take them very long to find your secret number.
Once they’ve found your secret number, they can now sign certificates pretending to be you. So someone could pretend to be your bank, and present a certificate saying “yes, I am your bank” with a signature on it that your computer trusts.
eMudhra sent you a padlock that had the code "4815" promising no one has ever used it or would be able to guess it, but all anyone really needed to try to get in is set the first number to "4" and ignore the rest.
It's not entirely accurate, but its accurate enough for them to get.