To be effective a guilty plea must be voluntary, unequivocal and informed. And to be informed, the person pleading guilty must be aware of the nature of the allegations said to constitute the offence, the effect of the plea;,and the consequences of the plea.
R. v. T.(R.) (1992), 10 O.R. (3d) 514 (C.A.), at p. 519;
R. v. Lyons,  2 S.C.R. 309, at p. 371.
A guilty plea is a formal in-court admission of guilt. It constitutes a waiver not only of the accused’s right to require the Crown to prove guilt beyond a reasonable doubt by properly admissible evidence, but also of the related procedural safeguards in the criminal trial process, some of which are constitutionally enshrined and protected.
T.(R.), at p. 519;
Korponay v. Canada (Attorney General),  1 S.C.R. 41, at p. 49.
On an appeal from conviction, an accused may challenge the validity of a guilty plea, but bears the onus of showing, on a balance of probabilities, that the plea was invalid because one (or more) of the elements essential to a valid plea was lacking.
In the usual course, an appellate challenge to the validity of a guilty plea entered at trial involves the introduction of fresh evidence in support of the claim, as well as a review of the trial record:
T.(R.), at p. 519.
An appellate court retains a discretion, exercisable in the interest of justice, to receive fresh evidence to explain the circumstances that led to the plea and to demonstrate that a miscarriage of justice has occurred.
R. v. Hanemaayer, 2008 ONCA 580, 234 C.C.C. (3d) 3, at para. 19;
R. v. Kumar, 2011 ONCA 120, 268 C.C.C. (3d) 369, at para. 34.
R. v. Faulkner, 2018 ONCA 174, at paras. 84-87.