Broadly speaking, the term "issue" refers to a person's lineal descendants.
Formally, California Probate Code Section 50 provides:
“Issue” of a person means all his or her lineal descendants of all generations, with the relationship of parent and child at each generation being determined by the definitions of child and parent.
Often, an estate planning lawyer will provide that a bequest will go to a beneficiary's "issue" if that beneficiary doesn't survive. It's also important to remember that the term "issue" is also used for purposes of intestate succession and anti-lapse statutes.
A Trustor also has the ability to broaden or narrow the definition of issue to ensure that the Trustor's estate planning documents properly reflect his or her wishes. Thus, a Trustor may wish to include adopted persons who were under a certain age at the time of adoption or to include stepchildren. (One may also exclude certain individuals.)