Constraints are the boundaries that the current state and future state must be defined within. 

For example, a client requests a new website to be built for their company, however, it must be built using a specific framework, such as WordPress. 

It must be built in such a way that the clients can maintain it themselves after delivery, and the development of the new website cannot take longer than 7 days.

This is an example of a technology, time, and skills constraint, which needs to be considered when planning the future state.