William Congreve wrote these words in the preface to his play The Way of the World. The play is an example of Restoration comedy, which tended to use stock characters (see e.g. Osherow). In the context of the quoted passage, Congreve also compares Terentius with Plautus (quoted from The Comedies of William Congreve, Volume 2, 1895):
Terence, the most correct writer in the world, had a Scipio and a Lelius, if not to assist him, at least to support him in his reputation. (...)
The purity of this style, the delicacy of his turns, and the justness of his characters, were all of the beauties which the greater part of his audience were incapable of tasting. Some of the coarsest strokes of Plautus, so severely censured by Horace, were more likely to affect the multitude; such, who come with expectation to laugh at the last act of a play, and are better entertained with two or three unseasonable jests than with the artfulness of the fable.
Congreve is not simply praising Terentius here but also anticipating reception of his play, which we would like to attribute to a lack of sophistication in his audience, which would supposedly not appreciate the subtlety of his work. The Way of the World, however, also uses stock characters, such as the intelligent and wealhty young hero (Millamant), the amorous elderly lady (Lady Wishfort) and the would-be wit (Witwoud). As it happens, the play's first performance was not very successful; it was only later that the play became Congreve's most famous work.
Congreve also discusses one of Terentius's predecessors, namely Menander:
As Terence excelled in his performances, so had he great advantages to encourage his undertakings, for he built most on the foundation of Menander: his plots were generally modelled, and his characters ready drawn to his hand. He copied Menander; and Menander had no less light in the formation of his characters from the observations of Theophrastus, of whom he was a disciple; (...).
This is yet another comparison that involves the quality of an author's characters. Kamm and Graham also discuss Terentius's characters (and his initial lack of popularity; cf. Congreve):
The comedy of manners effectively began with him. He was adept at employing the double plot, especially to illustrate different characters’ responses to a situation, and in developing the situation itself. There is also more purity of language and characterization than in Plautus, which may explain why Terence was not as popular in his own day as he would become later.
Based on the above quotes, it appears that "justness of characters" refers to the veracity of the characters in Terentius's plays.