What is the meaning of this sentence in Lolita?

Here is the paragraph from Vladimir Nabokov's Lolita:-

My task proved simpler than either of us had anticipated. Save for the correction of obvious solecisms and a careful suppression of a few tenacious details that despite “H.H.”‘s own efforts still subsisted in his text as signposts and tombstones (indicative of places or persons that taste would conceal and compassion spare), this remarkable memoir is presented intact.

I don't understand the meaning of the bold part. Specifically, what is meant by "taste" and "compassion" here?

Taste means good taste, and compassion what it normally means. The writer of the memoir wanted to avoid identifying people and places that it would be unkind or in bad taste to name, and the author (pretending that he was editing the character's memoir) says that he helped him to conceal them.