Most nouns can be either singular or plural.

We can have one pig or lots of pigs.

However, there are some words that cannot be pluralised. This means that they are always singular or always plural.

The noun trousers is always plural. We never say, 'My trouser is muddy'.

If we want to use trousers as a singular noun we have to say 'a pair of trousers'.

One noun that is always singular is bread. We never say, 'Put the breads on the table.'

If we want to talk about bread as a plural noun we have to say 'loaves of bread' or 'slices of bread'.