pent-up means “suppressed” or “held back.” Often the idiom is used to describe suppressed frustrations. In this piece, perhaps, it means holding back one's playful nature when performing abstract contemporary music. The title is also a pun – the materials are derived from pentatonic scales.
Recording by Bent Frequency Duo (Jan Baker, Stuart Gerber):
• Bent Frequency Duo, Atlanta, 4/7/16