What is a reporter enzyme and how can it be used to monitor transformation of host cells by foreign DNA in addition to a selectable marker?

As the name suggests, reporter enzyme is the enzyme which indicates or detects whether the foreign DNA has transfected host cells or not. 

They act as a selectable marker to determine whether the host cell has taken up the foreign DNA or the foreign gene gets expressed in the cell. The researchers place the reporter gene and the foreign gene in the same DNA construct. Then, this combined DNA construct is inserted in the cell. Here, the reporter gene is used as a selectable marker to find out the successful uptake of genes of interest (foreign genes). An example of reporter genes includes lac Z gene, which encodes a green fluorescent protein in a jelly fish.