How can I understand this "that"?

This is an excerpt from a book:
"Fears have also been expressed that justifiable sensitivity to the child's right to use his native dialect (regional, socioeconomic, or ethnic) within a national variety might lead to the abandonment of a national standard dialect and hence to the further disintegration of English."

I'm wondering how I can understand that "that" in "express that".
Could you tell me, please?
Is it conjunction or pronoun?