I am hoping this is simple

Famous last words! 🙂
Actually it is not bad. Just appreciate that computers were not designed to do what you are asking them to do. Mostly you need to add specific items to make the computer able to do what you need (and that is its strength); the fact that you can add these other items to make it do what you want. One of the specific items is a DRIVER. A Driver is a bit of computer code written by the company that built the device that "teaches" the computer what it is supposed to do with the information coming from this device.

We are not Tech Support for Focusrite, but will simply tell you that they (Focusrite, the company) recommend a Driver for their particular item (Focusrite 2i2) this driver will need to be installed on your computer. It will translate the information coming into the computer from the device. Please follow their instructions about versions and installation on your particular computer type and operating system.

The S90 XS audio LEFT/RIGHT Outputs connect to the Left/Right Inputs on the Focusrite 2i2 audio interface (use standard TS-TS signal cables)
Connect the Focusrite 2i2 to your computer as recommended in their documentation (its either USB or FireWire) - it will be obvious.
Also connect your Studio Monitor speakers to the main Left/Right Outputs of the Focusrite 2i2. The Focusrite connects to your speakers so that sounds coming from the computer can be heard.

It, the Focusrite 2i2, becomes the INPUT and OUTPUT device translating AUDIO coming from the S90 XS and routing it on to the computer. We highly recommend the FREE copy of Steinberg CubaseAI that came with your S90 XS, as your recording software. That is why it is provided.

This setup will allow you to record multiple tracks of data. You could enhance your piano tracks with strings, bass, drums, etc., etc. by overdubbing tracks one by one. You can even plug in a microphone and overdub vocal tracks... even building up multiple part harmonies, etc., etc. This is a very flexible and powerful home studio recording system.

That all said, if all you want to do is simply record some basic piano tracks... you don't need any of this stuff.

You have a Yamaha S90 XS!!! You can record audio directly to a USB drive connected to the "TO DEVICE" port on the back panel of the S90 XS or you can record directly to the S90 XS's 192MB of internal memory (about 18 minutes record time)

I believe continuous record to a USB drive is limited to 74 minutes per recording. Please see the S90 XS REFERENCE MANUAL page 131 for detailed step-by-step instructions on how to record yourself. The result is a 44.1kHz, 16-bit .WAV file that can be transported to your computer, burned as a CD, or emailed to a friend!

.wav files are a standard audio file format used throughout the world (Microsoft file format)... for audio, and is standard for burning CDs.

No messing with computer drivers, installation of software, or any of that, just plug and play (practically).

If you were told you needed an audio interface, that is true, if you want to record DIRECTLY to a computer.
BUT... if all you want to do is record some basic piano tracks, seems this is the easiest, quickest and most cost effective way. (Nothing additional to buy, nothing to do but plug in a USB drive, and play the S90 XS!) It doesn't get any easier than that.

Hope that helps