In this case, it means simply.(plain:simple/simply) When it is being used to emphasize facts that other people may not like to hear , plain usually occurs in the expression the plain fact / truth is that ...Simple can be used in this way too , but it can also be used in a wider variety of structures and collocations ( such as reason and matter )．

In this case, it means 'simply/ just' (plain: simple/ simply).