When you get sexually aroused, your vaginal walls fills with blood. This will push plasma from the cells in your vaginal lining, creating what is commonly called arousal fluid. This fluid is there to clean your vagina, create the right pH and lubricate your vagina to allow for easy penetration and for his semen to survive and make you pregnant.
You will often read in stories that the woman is 'wet' in love scenes. This is what it is. If you tell him that he made you wet, he should know what that means.
His penis does the same when he gets aroused. Per-ejaculate (often called per-cum) will come out of his penis to clean his penis and lubricate it to make it safe for his sperm when he ejaculates.
Please take care!