The old adage "the truth will win out" certainly applies to this play. Mrs. Alving has been keeping some dangerous secrets that cause a lot of pain when they are finally revealed.
Both of her secrets involve her son Oswald. The first concerns his health. Captain Alving was a cruel and unfaithful husband. She left him after only one year of marriage because of his infidelities. And it is because of those infidelities that Oswald has inherited a life-threatening. We're not told explicitly that his disease is syphilis, but the symptoms indicate that is what he is suffering from.
Like his father, Oswald himself is promiscuous. When his mother discovers him flirting with the maid, Regina, she knows she has to end their relationship. Regina is Oswald's half-sister.
As another famous playwright said: O what a tangled web we weave when first we practice to deceive!