If you are a small charity, and someone gives you a donation, you can usually spend it however you like, as long as it’s to further your cause, of course!
However, if someone asks for their donation to be used in a particular way, or even on a particular project, you obliged under charity regulations to honour that request (or if you can’t, you need to make it clear to the donor why you can’t). In other words, you have now received your first restricted income.
As you grow, you may find more of your income is restricted. You may want to encourage restricted donations. For example, if you send out a mailing asking for funds to be spent on young people in Scotland, you can’t very well spend the proceeds on building a hostel in Hertfordshire!
As you probably know, fundraisers refer to restricted income as being “earmarked”. Earmarking can relate to a specific project (e.g. re-housing tsunami victims in Sri Lanka) or can be very general (e.g. “Africa”, “youth clubs”).