Is Fast Aim / Reflex Training free?

Yes. Fast Aim / Reflex Training is released under Community / Steam Workshop and costs nothing. Neither it nor Counter-Strike 2 has any paid content.

What do I need to run Fast Aim / Reflex Training?

Counter-Strike 2, which is itself free. Fast Aim / Reflex Training is an add-on and will not run standalone.

The PlayBound Launcher installs it in one click from Counter-Strike 2's mods list. You can also install it by hand — full steps are on this page.

Roughly 38 MB, on top of the base Counter-Strike 2 installation.

Fast Aim / Reflex Training — Free Counter-Strike 2 Mod · PlayBound