When we count things in Thai, or mention a specific amount of something, we need to use measure words that groups words together in categories that we call classifiers.
Don't worry. It's not complicated at all. Just a different way of thinking. Once you get the hang of it, you will speak like a Thai in no time.
These are 3 very common classifiers taught in this video:
an : "piece", used for many different (usually small) objects.
kon : "person", used for any nouns refering to people.
dtua : "body", used for animals, clothes, furniture, letters of the alphabet and much more.