What is a fair-play mystery?

Here is how the book descriptions put it, in their own words.

A small town, a talking cat, a slow-burn police chief who drinks his tea black, and a whodunit you can actually solve. Every clue is on the page before the reveal. There is only one answer.
From the description of Fair Tea and Foxglove
Toffee and Yew is the second Hollow Oak Witch Cozy Mystery: a fair-play whodunit you can solve before the reveal, a slow-burn romance that finally gets a dinner reservation, and a talking cat who is, for once, not the oldest thing on the green.
From the description of Toffee and Yew
A fair-play cozy mystery: every clue is on the page, the contradiction is stated plainly, and you can solve it before Maggie does.
From the description of Wassail and Hellebore
Before the reveal, pause with Maggie and test the evidence yourself: which account cannot be true?
From the description of Maple and Monkshood
To find the truth, Maggie must ask what the evidence actually proves—and what someone hopes the town will believe.
From the description of Cranberry and Cowbane
She also has one account of a quarter to five that cannot be true.
From the description of Chestnuts and Mistletoe
Shortbread and Belladonna is the seventh Hollow Oak Cozy Mystery: a fair-play whodunit built from things you can check, a winter tea shop full of people who all saw part of the afternoon, and an amateur sleuth who learns that being sure is not the same as being right.
From the description of Shortbread and Belladonna
Every mystery is fair play: each turns on one contradiction stated plainly before the reveal, so the answer is always provable and never a guess.
From the description of The Hollow Oak Witch Cozy Mysteries: Books 1-3

Try one: start with Book 1