Montessori used this term to describe the purposeful activity of children

She observed that young children learn best when given the opportunity to engage in "hands-on” activities of their own choosing, as opposed to following the instructions of an adult.  

What Montessori preferred to call the "work of childhood”, some educationalists call "play”. What is certain is that children are serious when engaged in the kind of activity that meets developmental needs and, given freedom and time, will choose purposeful activities over frivolous make-believe ones.

[fa-link]  The Five Characteristics of Play--And of Montessori Work