What is the Cybersecurity Maturity Model Certification (CMMC)?

According to the DoD, the CMMC combines various cybersecurity standards and best practices and maps these controls and processes across several levels that range from basic cyber hygiene to advanced.
For a given CMMC level, the associated controls and processes, when implemented, will reduce risk against a specific set of cyber threats. The CMMC effort builds upon existing regulation (DFARS 252.204-7012) that is based on trust by adding a verification component with respect to cybersecurity requirements.
The goal is for CMMC to be cost-effective and affordable for small businesses to implement at the lower CMMC levels. Authorized and accredited CMMC Third Party Assessment Organizations (C3PAOs) will conduct assessments and issue CMMC certificates to Defense Industrial Base (DIB) companies at the appropriate level.
Source: DoD Department for Acquisition and Sustainment - https://www.acq.osd.mil/cmmc/