An uncountable plural is a plural (i.e. it has grammatical number of 2 or more) that is an uncountable noun (i.e. we cannot differentiate grammatical number). These are defective nouns in the sense that they don't have singular forms that can be used as nouns.
In English most of these are dual, i.e. they have a grammatical number of 2. For example "scissors", "trousers", "pants". They can be referred to with a "pair of". When used as noun modifiers, they usually are in the singular form, e.g. "trouser leg", "scissor blade", etc, not *"trousers leg" nor *"scissors blade". Scissor kick seems to be an exception -it is often "scissor kick" or "scissors kick".
The other main example is the word "clothes", which has a grammatical number of ≥2. If we want to count clothing we say "[number] item(s) of clothing". As a noun modifier, this remains plural, e.g. "clothes shop", not *"clothe shop". Also uncountable plurals are some types of clothes, such as "pyjamas" and "overalls".
As plurals, they must use plural verb forms, particularly "are"/"were" and are not inflected with a third person -s.
Contrast countable plural.