"tlheHmeyna' reSlaHbe' 'IDnar lIlwI'pu'."
Translation:Stage magicians cannot cast real spells.
From The Klingon Dictionary:
"It indicates that there is no doubt in the speaker’s mind as to the accuracy of his or her choice of words."
So it means that the speakers is saying "spells is the right word and exactly what I mean - not tricks, not science, not illusions, but spells." I suppose "genuine" is a decent approximation, as well as "real" ir "actual".
As Jeremy says, the dictionary meaning of -na' is to affirm that the speaker is positive about the identity of the noun, and we have the canon sentence from The Klingon Way tIqDaq HoSna' tu'lu' Real power is in the heart to support the way it's used here.
What it does is underscore that whenever anyone uses the"no true Scotsman" fallacy it's just their opinion, e.g. wa'vatlh SochmaH SImyon SIchpa' Hat bIr, yopwaH bID tuQtaH qa'naDa'nganna'
Real Canadians wear shorts until cold temperatures reach fifteen below.