You are given the options of Fast, Good, and Cheap, and told to pick any two. Here Fast refers to the time required to deliver the product, Good is the quality of the final product, and Cheap refers to the total cost of designing and building the product. This triangle reflects the fact that the three properties of a project are interrelated, and it is not possible to optimize all three – one will always suffer. In other words, you have three options:

Design something quickly and to a high standard, but then it will not be cheap.
Design something quickly and cheaply, but it will not be of high quality.
Design something with high quality and cheaply, but it will take a relatively longer time.