An exclusive game is a game available only on one platform, or on one platform owner set of platforms, and not on direct competitors. Exclusivity can be permanent, timed until a set date, or limited to consoles while also appearing on PC.
Exclusives exist because platform owners need reasons for players to choose their hardware. A game you cannot play anywhere else sells consoles in a way a shared game cannot, so owners fund, buy, or contract exclusivity and feature those games at the centre of their marketing. For developers, an exclusivity deal can provide funding, marketing support, and a guaranteed audience in exchange for giving up sales on other systems.
The terms vary and the wording matters. A full exclusive never officially leaves its platform. A timed exclusive launches on one platform first and arrives elsewhere after months or a year, a fact sometimes only confirmed when the second version is announced. A console exclusive is exclusive among consoles but also sold on PC, which is now the most common arrangement for large publishers.
Players treat platform availability as a fact about access and cost, not as a quality judgement. An exclusive can be excellent or poor, and its status mainly decides what hardware a player needs to buy to play it, which is why changes, such as a former exclusive being ported elsewhere years later, are reported as news.
In play"You will need that console to play it at launch, because it is a timed exclusive for a year."