Use this function with the = and in() operator to find if a specific item has been checked. For example:
"Definition of Done" = checkedItem("Peer Review")
"Definition of Done" in (checkedItem("Peer Review"), checkedItem("Code Review"))
returns all the issues where the Definition of Done has Peer Review checked. Note that this is an exact search and hence, it is case sensitive. Also, using the != operator does not return the issues for which Peer Review has been checked. If you want to search for an item which is not checked, use the uncheckedItem function instead.