Something or someone is ugly if they make you feel bad when you look at them. The opposite of ugly is beautiful.
Underdone describes food that hasn't finished cooking. Sometimes food is underdone on purpose.
A person or animal is underweight if they weigh less than they should. When you are underweight you can easily see your bones.
You are undressed when you are wearing either no clothes or you are not wearing enough clothes. The people in the picture are undressed. When you are taking your clothes off, you are said to be getting undressed. The opposite of undressed is dressed.
Something is unfinished if it has not been completed yet. A similar word to unfinished is incomplete.The opposite of unfinished is finished.
Someone is unfit if they are in poor physical condition, and get tired very easily when they do physical activity. The man in the photo is very unfit. Unfit also means something that doesn't suit a purpose, for example a rotten fruit is unfit to eat.
Unfortunate is another word for unlucky. Unfortunate means that something bad has happened that is out of your control. The opposite of unfortunate is fortunate.
Something is uniform if it is the same everywhere. A wall that is all one color might be described as being a uniform shade of green. A uniform is also the name given to the clothing all people in an organisation have to wear so they all look similar. People in the military wear uniforms, and many schools require that children wear uniforms. Uniforms for men and women are often quite different, even in the same organisation.