George Bernard Shaw's Candida as a problem play; note the following the definition—it is a kind of...

...drama that developed in the 19th century to deal with controversial social issues...and stimulate thought and discussion. [For example,] Henrik Ibsen...exposed hypocrisy, greed...in a number of masterly plays. His influence encouraged others to use the form. George Bernard Shaw brought it to an intellectual peak with his plays and their long, witty prefaces.

Candida's purpose is not to expose social ills (though it does casually refer to "socialism and corruption in government"); it deals more with "controversial social issues" such as the reality of marriage in the 19th Century.

Candida is a play of ideas.

[Shaw] has been credited with creating the “theater of ideas,” in which plays explore such issues as sexism, sexual equality, socioeconomic divisions, the effects of poverty, and philosophical and religious theories.

Some of Shaw's critics often find...

that his plays are merely tracts for expressing Shaw’s ideas on love, war, property, morals, and revolution...

...but not with regard to Candide. Whereas many of his plays deal with social issues and rebellion, this play deals mostly with the old-fashioned institution of marriage. Morell is married to Candida; Marchbanks is a young, unrealistic poet who idealizes Candida and admits to Morell that he loves Candida. The idea here is: what is love? It ends up being about marriage, not the romantic ideas of Marchbanks.

Whereas Marchbanks (eighteen years old) is part of generation of young people who do not look at the world realistically—but with the "rose-colored glasses" of idealism—Morell is grounded ("mentally and emotionally stable: admirably sensible, realistic, & unpretentious") as a clergyman, husband and father. Marchbanks is appalled that Candida is reduced to menial tasks within the household (like peeling onions), but Candida is much more sensible than Marchbanks; she easily assumes her role as wife, mother and homemaker. She may feel a friendly affection for the young man, but nothing else.

On the other hand, Morell, having listened to Marchbank's poetic spoutings, wonders if he is not, indeed, too "mundane" for his wife. However, Candida is presented as a strong woman who is happy to support her husband and do what she can to see to his success.

Marchbanks demands that Candida decide—she chooses "the weaker one,"—her husband. He needs her support and faith in him. He is not presented as a weak figure, but as a man who succeeds because of his wife's dedication. The two have a solid give-and-take relationship that allows them to be happy with their marriage and love for each other. Their relationship is not the stuff poetry is made of, but realistically, life generally is not.

Marchbanks leaves: he has the soul of a poet—searching for places where he might change the world. The "mundane" life of marriage is not for him.

Many regard this, a husband and wife coming to a fuller understanding of each other, as the central aspect of the play.

Candida may be seen as a play of ideas in that it is philosophical: comparing the views of idealistic and inexperienced rebellious youth to the more realistic experiences of those working through life's challenges, such as marriage, which require dedication and devotion.

Additional Source:

http://www.merriam-webster.com/dictionary/grounded