I must admit that it is hard to pinpoint "the true meaning" of any work of literature just because there are many themes that can be found within them. Lolita is no exception. However, if I had to choose one main theme, I would say that it would be appearance vs. reality: the appearance of Humbert as an artist, but the reality of him as an obsessive pedophile.
There is an absolutely perfect quotation to exemplify this theme:
The majority of sex offenders that hanker for some throbbing, sweet-moaning, physical but not necessarily coital, relation with a girl child, are innocuous, inadequate, passive, timid strangers who merely ask the community to allow them to pursue their practically harmless, so-called aberrant behavior, their little hot wet private acts of sexual deviation without the police and society cracking down upon them. We are not sex fiends. We are unhappy, mild, dog-eyed gentlemen, sufficiently well integrated to control our urge in the presence of adults, but ready to give years and years of life for one chance to touch a nymphet. Emphatically, no killers are we. Poets never kill.
Humbert prefers his appearance as an "artist." Above Humbert calls himself a "gentleman" who is "sufficiently well integrated" and able to control his urges "in the presence of adults." Note also that Humbert ends with "poets never kill." Referring to himself with words of artistry helps Humbert to justify his more base and disgusting behaviors in his own mind. It makes me think of the fine line (or is it a huge gray area?) between artistic nakedness and pornography.
Of course, while Humbert appears to be an artist, the reality is that he is an obsessed and deranged pedophile. In the quotation above you can see the wording proving his reality with words like "throbbing" and "sweet-moaning" and "nymphet." The fact that Lolita is actually Humbert's step-daughter further confounds the situation. Further, there is damning information in Humbert's statement about love:
I knew I had fallen in love with Lolita forever; but I also knew she would not be forever Lolita.
The reason "she would not be forever Lolita" is that she will not always fit his obsession with pre-pubescent girls. Eventually, Lolita will grow up and become a woman. At that point, Lolita will no longer be interesting to Humbert.
There are a couple of ways that, in conclusion, we can go even further with the theme of appearance vs. reality. For example, Lolita also can apply to this theme. She has the appearance of innocence (from Humbert's own words), but she also seduces Humbert and eventually demands money for sexual activity. Also, the frame of the story nicely frames the theme as well. The story is written as a "confession" from Humbert. Therefore, the reality has been hidden since this time by appearances. Further, the confession also had to be "edited" by someone else. The horrors of the truth might still elude us as readers!