Suppose you make a flop check-raise ONLY with TPTK (top pair, top kicker). Your play is unbalanced, easily readable.

A balanced range includes both strong, medium and weak hands to keep our opponents off balance. Another term you'll hear is "polarized range." That's a range that only includes very strong and very weak/bluffy hands.

All that said, if you're at 25nl and below, the concept probably doesn't have much application to your game.