A firewall is a quiet piece of security that sits between your PC and the network, deciding which connections are allowed. For online gaming it usually works invisibly — and occasionally it is the reason something will not connect.
What a firewall does
A firewall controls network traffic in and out of your PC. It can allow or block connections based on rules — which programs may reach the network, which incoming connections are accepted. Windows includes one, your router includes one, and the point of both is the same: not everything that wants to talk to your PC should be allowed to.
How games interact with it
Online games need to make connections — to game servers, sometimes to other players. Most of the time this just works: a firewall typically allows the outgoing connections a game starts, and a well-behaved game asks for any permission it needs the first time you run it, which is the prompt you may have clicked "allow" on without much thought.
When the firewall gets in the way
Problems show up at the edges. A game may fail to connect, host a session, or use voice chat if a firewall is blocking what it needs. Strict network setups — and the layered effect of a router firewall plus the Windows firewall plus sometimes a third-party one — can interfere with games that need incoming connections. The symptom is usually a connection that fails for no obvious reason while the rest of the internet works fine.
The right way to fix it
The instinct is to turn the firewall off. Resist it. A firewall off is real exposure, and "disable your firewall" appearing in instructions from an unofficial source is a warning sign. The correct fix is specific: allow the particular game through the firewall — Windows lets you permit an app by name — rather than opening everything. If a game documents the connections it needs, allow those. Targeted permission keeps the protection while removing the obstacle.
The takeaway
A firewall decides what may connect to and from your PC, and for gaming it usually works invisibly. When it does block a game, the fix is to allow that specific game through — not to switch the firewall off. Targeted exceptions solve the problem without trading away the protection.