Answer: She says that Lady Harbury looks younger since her husband’s death.
Step-by-step explanation:
To make light of something means to imply that is is not so serious or should not be considered seriously.
In this excerpt, Lady Bracknell comments on how Lady Harbury looked twenty years younger since losing her husband. This can be taken to mean that she was younger because her husband was dead and the marriage over.
One can infer from this that marriage should not be taken so seriously because it can drain a person such that they will look older than they are supposed to.