The way I understand is ‘bolna’ is synonym with ‘utter’ or ‘speak’ in English. Hence, for animals people use this word.
In human ‘bolna’ becomes interchangeable with ‘kehna’ (saying) since we understand their language.
Now if a person does not make sense , then people may say ‘bolna’ word even for human. Not so strong difference though.
Examples:
“Bhai Yeh Kia Bol Raha hai?”
“Lagta Hai Yeh Aadmi Kuch bol raha hai”
“Are Kuch to Bolo” Meaning even if you do not make sense, say something.
Now your example “to explain” or “to Tell” is more close to “batana” “batlana”
“samjhana”.