What is std? I don't understand.

Sexually transmitted. An std is an infection transmitted by sexual contact, including vaginal, oral or anal intercourse. There are many std's, including viruses, bacteria, and even some parasites.
Infections that are. Spread through sexual contact. Examples: chancroid, lymphogranuloma venereum, mycoplasma genitalium, trichomoniasis, crabs/pubic lice, scabies, herpes, human papilloma virus (hpv), hepatis b, hepatitis c, hiv/aids, chlamydia, gonorrhea ; syphilis.