"CD" in this context usually stands for 'Cumshot' or 'Cream Pie'. It refers to a scene where the performer ejaculates on or inside a sexual partner, creating a "cum shot" or a "creampie."
"Sucks" is a common term used in pornography to describe oral sex, specifically when one person stimulates another by using their mouth. The phrase "sucks BBC" indicates that the person giving oral sex is focusing on a partner with a large penis, often referred to as a 'big black cock' or simply 'BBC.'
So, the tag "cd-sucks-bbc" means a scene involving oral sex performed on a partner with a large penis, resulting in ejaculation inside or on the recipient.