lump near left testicle

My husband is 25 yrs old,he went to the doctor today because he had found a lump about the size of a marble that makes him feel like he has 3 testicles.The doctor said it felt like a lesion but was very hesitant and was quiet for a few minutes and then made him an appointment for an ultrasound.When my husband told them he was worried about it being cancer,the doctor said there was a small chance but he would be okay and left the room.I don't understand any of it and neither does he.What exactly is a lesion?Is it cancer or dangerous?Please someone help