The number of a noun or pronoun is either singular, if it refers to one thing, or plural, if it refers to more than one thing (if the noun or pronoun is the subject, then its number will also affect the verb). Note the difference in number in the following examples:

Singular

That woman is concerned about this issue.

She is concerned about this issue.

Plural

Those women are concerned about this issue.

They are concerned about this issue. 

It is important to note that the pronoun "they" is in the process of becoming singular as well as plural. For example, one might say

A person called and they did not leave their name.

This construction allows the speaker to avoid identifying the gender of a person, and it has been common in speech for decades, if not for centuries. Be aware, however, that some people still consider it unacceptable for formal writing.

For more information, see noun plurals.