The Greatness of Saying Tehillim Even Without Understanding The Words:
The Medrash (Socher Tov on Tehillim 19:15) states: “Yihiyu l’ratzon imrei fi.” (May the words of my mouth be to Your liking.) Dovid Hamelech requested of Hashem that reciting Tehillim should become ingrained in people of all generations. It should not be like reading a sefer of mussar. Rather, it should be as if one immersed himself in learning Negaim and Ohalos, and one should receive reward accordingly.
The simple explanation of these words is that even if simple people say Tehillim, they should the same reward as great Torah scholars who plumb the depths of the most difficult areas of Torah. However, Rav Mordechai Dov of Hornesteipel zy”a (son-in-law of the Sanzer Rov zy”a) explains differently. He quotes Rashi on Parshas Tazria that says that negaim only become tamei or tahor after being declared so by a kohen. The source for this is the Toras Kohanim (13:49) that says that even an unlearned, ignorant kohen, who is not well-versed in the halachos, can declare a nega on a Yisroel who is a talmid chochom to be tahor, but the Yisroel talmid chochom, who knows all of the halachos fluently, cannot do so for himself.
With this in mind, he explains that Dovid Hamelech was requesting of Hashem that one who reads Tehillim, even if he is an ignorant person, should have the same power that a kohen has in the area of negaim and taharos. He should be able to purify the impure and rectify the world, even though he is not a learned person.
