What is a freeware game?
This one is easy. A freeware game is a game available for free, no payments are necessary. It may seem a little strange to someone used to buying games on Steam (maybe during Steam sales...), that the internet is already stuffed with A LOT of completely free video games, but it's true!
Not every free game is a high quality game, we're talking about software often developed by a single person, maybe as a way to learn a new programming language, so a lot of free games are, well... there's no other way to put it... plain horrible, but you have to respect the effort. After all, you are not paying a dollar (or an euro, or pound, or... whatever).
There are also some free-to-play video games that you can download and play right away but are filled with optional content that you can buy. There is a subtle difference between completely free and free-to-play games, learn more about free-to-play games.
Last but not least, freeware doesn't mean that you can do anything with it. For example some freeware games can't be re-distributed without the owner's permission.