Accreditation is a quality assurance process that colleges, universities and education institutions or programs undergo to confirm that they meet a strict and recognized set of service and operational standards.
Accreditation is assessed by private, nongovernmental accrediting agencies that have been created specifically to review education institutions and programs. These external college accreditation agencies are reviewed and recognized by entities such as the Council for Higher Education Accreditation (CHEA) along with the United States Department of Education (USDE), which hold quality standards of their own.
In the United States accreditation is voluntary. Colleges do not need accreditation in order to open their doors or offer educational programs. This is an important distinction you need to make if seeking a college. If you want to avoid degree programs that have been denied or have opted out of accreditation, it's up to you to research these facts.

The importance of accreditation is that it creates a set of quality standards for all education institutions or programs, provides access to federal and state funding, maintains private sector confidence, and makes transferring credits easier. Furthermore, accreditation aims to ensure accountability of schools and degree programs which boosts public trust and confidence.
When an institution or degree program is properly accredited, you are able to gauge its overall quality without having to conduct a detailed analysis on your own.