Backdoors are like secret entrances to programs that allow the creators or the people that know about the backdoor to bypass any security and encryption.
Such gateways make data, that should be available only to its users, accessible to anyone that knows about that secret entrance.
The difference between a backdoor and a zero-day is that backdoors are made with a will to bypass security. Zero-days are mistakes in the software that leads to security being circumvented.
To create a backdoor, you need a high level of sophistication. Not only you need to hide it in the software, but you also need to make it available for the people that know about its existence.
It’s also probable that in your lifespan you have been using at least one software or hardware with backdoors. The only difference is that you might not have been the target of an attack via that open door.