.props() is just another builder method on model types that allows you to add or override props given to a model.See docs.

constSomeModel=types.model({firstName:"Tyler"}).props({lastName:"Williams"})constmodelInstance=SomeModel.create()console.log(modelInstance.firstName)// Tylerconsole.log(modelInstance.lastName)// Williams

More useful if you're extending an existing model, in my opinion. But some people useprops instead of the initial properties argument to thetypes.model function.