The idea behind an undercut is to eliminate the weight completely from a certain section of hair. It can be drastic by shaving part of your hair or more subtle by using a razor, scissors, or thinning shears to make a section of hair dramatically lighter so it's practically non-existent. Usually the rest of your hair will lay over the section and cover it while having more movement because the hair that usually supports it is gone.
An undercut is definitely synonymous with 90's grunge and the best part of it is that it can be a secret from the world or out for everyone to see. Although searching"undercut" can result in endless photos of acne ridden teenagers and shaved sides, Grimes' undercut pictured above is the perfect definition of it for me.