If you are chuffed, you are very pleased — happy with how something went, and a bit proud of it. “I’m chuffed” is British informal speech; Americans almost never say it. British speakers pile it up: chuffed to bits, dead chuffed, well chuffed. It is the sort of thing you say when you passed a test, your team won, or someone liked your cooking — pleased, and not hiding it, but not jumping up and down either. It comes from a northern English dialect word chuff, which first had to do with looking puffed and well-fed (not a Scottish word). The “very pleased” meaning is not old in print; it turns up in the 1950s. One messy bit, said kindly: in a few older dialect uses, chuff / chuffed could mean the opposite, a bit sulky. You can forget that for modern English. If a British friend says they are chuffed, they are happy.