Devolution is a process of decline. Something has gone from great to good, good to bad, or excellent all the way to horrible.
If you know that evolution refers to how smaller or lesser creatures evolved into more complex beings, than you have a great clue to what devolution means. When devolution occurs, things have gone in a negative direction. If a rich person loses all their money, that's devolution. If a championship basketball team suddenly has a losing record, that's devolution. This word also refers to a central government delegating authority to a smaller body, such as a regional government.