"cat agus cù beag" - a cat and a small dog. There is no verb so beag goes with cù
"Tha cat agus cù beag" - the verb "tha" forces "beag" to apply to the subject/noun phrase of "cat agus cù".
If you wanted to say, e.g., "a cat and a small dog are eating", you'd use "tha cat agus cù beag ag ithe" - more literally, "a cat and a small dog are at eating". It's all about having is/are in there with "tha".
It's because there are no articles ("an", "am", "an t-", etc) in the sentence, "Tha cat agus cù beag." "tha" is the verb and means "is"/"are".
There isn't an indefinite article in Gaelic so Gaelic "cat" means both English "cat" and English "a cat". To say the English "the cat", you'd use the Gaelic "an cat".
So "a cat and a dog are small" is written "tha cat agus cù beag".
To say "the cat and the dog are small" you need another word - the article that identifies a specific dog and specific cat. That is when the sentence changes to "tha an cat agus an cù beag".
The course will cover it when you get a little further down, but if you want to read about it in the notes you can find it starting under "masculine article" on this page: https://www.duome.eu/tips/en/gd
I absolutely made that more confusing with an example I used above so I've edited my original reply; hopefully I didn't confuse you too badly, I'm sorry!