The noun dress is countable. It means the long outer garment worn by a girl or woman. It goes from the shoulders to below the hips.

She pays too much attention to her dress.
I have never seen you wearing a dress.

There is also an uncountable noun dress. It means ‘clothing’ or ‘clothes’. Note that the uncountable dress cannot be used with the article a/an. As an uncountable noun, dress is mostly used to talk about special kinds of clothing. Examples are: evening dress, national dress etc.

Everybody is required to wear their national dress on the occasion. (NOT Everybody is asked to wear their national dresses…)
He looks good in evening dress. (NOT He looks good in an evening dress.)

Dress as a verb

The verb dress is used to talk about putting clothes on oneself or somebody else. Undress is the opposite of dress.

She takes hours to dress in the morning.
She undressed herself. (= She took her clothes off.)

In an informal style we often use the expression ‘get dressed‘ to talk about dressing oneself.

Get dressed in five minutes.

When an article of clothing is mentioned we use the expressions put on and take off.

She put on a coat.
He took his boots off.

Be dressed in

The expression be dressed in is used to say what somebody is / was wearing on a particular occasion.

He was dressed in a dark suit.

The expression be wearing can be used instead of be dressed in. This is very common in British English. In American English, the expression have on is more usual.

She had on white pyjamas. = She was wearing white pyjamas. = She was dressed in white pyjamas.