Claiming exemptions.

Hi. I’m new to everything taxes and am anxious to know the answer to this. I am single and claimed 0 exemptions for most of the year. However for 4 months only, I claimed 3 exemptions. When editing my W4 online it was the only box I was given to change. Will I get in trouble? What will happen? What’s the difference between exemptions, allowances, and dependents. I am single with no children. Help please!