Open source means the source code is published and anyone can read, modify and redistribute it under a licence. That sounds like a developer concern and it has consequences for ordinary users.
Free is not the point
Most open source software costs nothing, and that is a side effect. Plenty of free software is not open source, and some open source software is sold commercially.
The substance is that you can see what it does and nobody can take it away.
Why that matters practically
Continuity. If the company behind a closed product shuts down, the product stops. If the company behind an open project shuts down, the code remains and someone else can continue it. That has happened many times.
Verifiability. Security software that claims not to send your data somewhere can be checked. Claims about closed software have to be trusted.
No forced changes. A closed product can add advertising, change pricing, or remove a feature. An open one can too, and you can use the previous version or a fork indefinitely.
The honest downsides
Support is often community-based rather than a phone number. Polish varies widely. And some categories have no competitive open alternative.
Where it is worth choosing
Anything you want to still work in ten years. Anything handling sensitive data. Anything you would be stuck with if the vendor changed direction.
The part nobody sees
Most of the internet runs on it. The servers, the databases, the encryption libraries, the operating systems. It is not a niche; it is the foundation, maintained largely by volunteers.