É um núcleo extra usado quando a CPU de maior potência (4 núcleos) não é necessária.
PCMag
Nvidia has settled on a name for the unique processor architecture used for its quad-core Tegra 3 chip – 4-PLUS-1. The "PLUS-1" refers to the mobile processor's fifth core, a "battery-saver" core that runs on very low power and executes tasks at low frequency when the chip's more powerful and power-hungry ARM Cortex-A9 cores aren't needed.