Translation:Tim is a dog, he does not understand Russian.
The dash is used instead of a present tense verb "to be." Мой отец -- врач. My father is a doctor. Technically they don't really even need the dash, and you'll be marked correct if you don't use it. Grammatically the dash is unnecessary because "Tim" is in the nominative and "собака" is also in the nominative, so I'm not too sure why the dash is used, either, or how a dash was chosen, other than maybe stylistic choice. Regardless, I haven't ever seen Russian use conjugated "to be" present tense outside of quotations of Christ directly from the Bible, or religious-like language. Maybe a native speaker would like to give us a history lesson?