High unit junior means 80-89 semester units, or 120-134 quarter units. Senior is 90 or more semester units, or 135 or more quarter units. Note that AP/IB/A-level units are not counted for this purpose.
https://www.ucdavis.edu/sites/default/files/upload/files/f17tip_unitlimits.pdf has some examples.
What is the problem with cognitive science at UCSC?