Translations into English:
Suffix appended to words to create a masculine noun, usually denoting a performer, feature, human relation, result of an action, object, diminutive or a proper name.
Suffix appended to the present stem of verbs to form an adjective denoting a feature or a dimension.
glad (“to smooth”) + -ak → gladak (“smooth”).
ljub (“to love”) + -ak → ljubak (“amiable”).
Proto-Slavic *plyti + -ak → plitak (“shallow”).