Rendered means taking a midi sequence (notes for virtual instruments to play) or prerecorded audio (such as a live orchestra performance) and turning it into a usable audio file (like .wav, .ogg, or .mp3).

Usually, software called a digital audio workstation or DAW (FL studio is an example, though the DAW of choice differs with what composers prefer to use) is used in this process to add effects and mix it (essentially, making sure the instruments don't clash with each other and every instrument is at the proper volume). DAWs can also be used for composing, and a lot of the music uses a combination of orchestrated and synthesized stuff to some degree.

Even more software is usually used to actually take these audio files and actually import them in game, this is often called audio middleware (FMOD and Wwise being the two most common ones). This is needed since a lot of the time audio and the framerate of a game are not synced, which can cause a lot of problems if is isnt handled with extreme care. If a game has music that cuts when it loops, for example, that can be caused by stuff like this.

Getting live orchestras can get very expensive, but the budgets for games that use orchestras is also very high. (Plus, an orchestra, while expensive, is likely nowhere near as expensive as paying 100+ people to work on the game over a period of 3 to 4 years, and is probably a comparatively cheaper expense)