I've played this game for a little while, but I'm still not entirely sure what each of the stats do, and I can't find any resources telling me what they mean. The only things that I've worked out were that a higher drift gives you tighter turns while braking and that the board is the character whose stats (at least as far as drifting is concerned) are used.

First of all, the stats on the character select screen don't tell the full story. There are some very impactful stats that are hidden in the code, and you can see full stats here: https://docs.google.com/spreadsheets/d/1_5_5GcfLbVVIfoqVaIuVjwSsrpfQgmM3UTaCicZD8n8/edit#gid=2000748679 It's slightly out of date because stats are handled server-side now, and can no longer be data-mined.

Now for each stat: Speed - Top ground speed, hard to determine how much this matters due to no speedometer

Air - Top air speed, correlates strongly with the hidden "Elevator" stat that determines how much air you can get and how quickly you fall

Boost/Acceleration - How quickly you can accelerate and how much of a speed boost you get from switching or going through rings. Boost lets you go above your max ground/air speed. This is one of the most important stats.

Drift (Cornering & Grip) - The drift stat on the character select screen is a combined representation of cornering, which is how well you can turn while drifting, and grip which is how well you can turn without. These two values are pretty close for most characters, and most people find it easier to control characters with higher cornering than grip, even when both are low.

Mass - Only used in multiplayer, affects how much you knock others away when colliding/how little you get knocked.

Spell - Determines how quickly your meter fills, and how much meter you get from rings.

Two important stats that are hidden are drift slip and drift add.

Drift slip determines how far you slide out when drifting. Compare Suika to Clownpiece for an example of how much difference this makes. Both have bad drift, but Suika slides way out, whereas Clown hardly slips at all. A high drift slip is harder to control, but is generally agreed to be faster than low drift slip.

Drift add is kind of hard to explain, but it affects how much you can alter the course of your drift. Most speedy and powerful characters have 0 for this. All technicals have 3. Cirno and Yukari have 1, and not other values are used.

All stat values come from the board character, but the rider is used for the last word pulls in multiplayer. Both characters LW meters (yellow bar) charge simultaneously, and the charge time varies by character. When using double of one character, the charge time is the same, but pulling a last word will deplete both meters.