Fine-tune definition is – to adjust precisely so as to bring to the highest level of performance or effectiveness.

The fine-tuned Universe is the that the conditions that allow life in the Universe can occur only when certain universal dimensionless physical constants lie within a very narrow range of values, so that if any of several fundamental constants were only slightly different, the Universe would be unlikely to be conducive to the establishment and development of matter, astronomical structures, elemental diversity, or life as it is understood. Source: Wikipedia