Rashi: I know that there is a good reward in the world to come for those who fear You. Even so, in this world, that Resha'im surround them, I pray for them, that "You hide them in the hidden place of Your face (desire)" (verse 21).

Radak: Resha'im speak with pride and contempt about Tzadikim and Yir'ei Hashem, for they see them lowly and weak. They do not know how great is the good that You hid for them in the world to come! If they are lowly in this world, they will be elevated in the world to come.

Radak: Aside from the great good hidden for Tzadikim in the world to come, You often gave great good to them in this world in front of people, lest they be shamed in their hearts because they trusted in You.

Malbim: The great good for Tzadikim in the world to come, it is hidden; people do not see it. The good that You do for those who trust in You is not hidden; it is in front of people. All see it. (I.e. we see evil befalls Tzadikim in this world. There is an answer for this - there is great reward [for them] in the world to come. However, this is hidden, and many deny Hashgachah, for it seems that evil befalls Tzadikim. The Hashgachah that Hashem does for those who trust in Him is open to all, and silences the deniers (verse 19).