Honda officially calls these the "First-2nd Select Spring" and "Fifth-Sixth Select Spring."
They are the parts in your transmission that are (partially) responsible for your shifter centering itself when in neutral. By increasing the rate and preload of these springs, it's possible to realize a more decisive centering action for a sportier shift. ACUITY's K-Series Transmission Performance Spring Upgrade uses thicker wire and additional preload to provide a very satisfying, quick centering motion. This mod is particularly attractive because it provides such a noticeable change at a rather low price.
Fitment:
Most K-Series manual transmissions
DOES not fit the 2017+ CTR or 2018+ Accord
Transmission Springs #1887 Install Guide