What is Open source?

Open source means the software is published so anyone can read the code, check what it does and build on it, which matters when a service asks for trust. addy.io is open source, and so are AdGuard and AdGuard DNS, whose public servers and clients can be examined directly. 1Password is not open source, and AdsPower is not open source either, so neither lets readers verify its internals the same way. Open source is not a full security judgement on its own, but it does change how much independent scrutiny a tool can attract.