The corona (also known as corona of glans penis) is the protruding circumference at the base of the glans (or head) of the penis. The corona is visible on uncircumcised penises or when the foreskin is pulled back. The corona separates the glans from the shaft.
More About Corona
The corona is one of the most sensitive parts of the penis, along with the frenulum. As it forms the base of the glans, it is an easy part to identify and stimulate, especially through oral sex. Men find stimulation of the corona especially pleasurable. The corona (like most of the glans) is usually hidden under the foreskin, unless the foreskin has been pulled back or the penis is circumcised.