Constraints are conditions you expect the design to meet. These can be requirements specification (translated into formal language), or specific queries to check behaviour such as output bounds or logic/temporal conditions.
When writing a constraint on the fly, you can click on any specific input, output, or wire on the model to place the corresponding parameter where the cursor is on the constraints window. This makes it very fast to build queries and conditions. Constraints for a specific model can also be saved and loaded in a text file with the .QCT extension.
You can learn more about the constraint language in the QCT language section in this guide.