What is endometrioma? is that cancerous? what does having a chocolate cyst on the left ovary mean?

Not cancer: Endometriosis is a common problem in women when benign endometrial tissue can be found elsewhere. It can cause bleeding, pain, scarring, etc. When endometriosis forms a cystic mass (such as in the ovary) it is referred to as an endometrioma. A chocolate cyst is just a synonym that sounds "less medical" and refers to the (perhaps) "chocolatey" appearance of the cyst contents.

Benign: Endometrioma refers to a benign cyst that forms in the ovary when a piece of endometrial (uterine) tissue attaches to the ovary. Because the endometrial tissue inside the cyst responds to monthly hormones it bleeds and fills the cyst with unclotted blood. They are also referred to as 'chocolate cysts' because the dark old blood inside the cyst grossly resembles chocolate.