This porn video tag can be broken down into three main elements: "big", "cock", and "cum".
1. "Big": Refers to a large size, in this context it is likely referring to the size of the male performer's penis being larger than average.
2. "Cock": A slang term for a male's penis. In porn video tags, this word is often used instead of the more explicit terms to avoid censorship.
3. "Cum": This refers to the act of ejaculation, specifically when the male performer reaches orgasm and releases semen from his penis.
So, this tag would be describing a porn video featuring a scene where a male performer with a large penis is engaged in sexual activity leading up to and including ejaculation.