Technically, there is no such thing as critical constraints, a constraint, by definition is critical to the project.
However, in some organizations, critical constraints are defined as those constraints with a weight of 0.4 or more (so you can't have more than two critical constraints in the same project).
An example of the above is a project that is has a very strict budget and a deadline. The project needs to be finished by the end of the year and can never surpass the budget (let's assume because it will bankrupt the company). Thus, the weight of the cost constraint is 0.5, the weight of the schedule constraint is 0.4, and the weight of the scope is 0.1. In this project, both the cost and the schedule are considered to be critical constraints, as it will be catastrophic to the project if they are exceeded.