How to understand the high-mode in TEM cell?

Hi,
I measured the probe in a TEM cell. The highest frequency of the Cell is 300MHz.
It is said when the frequency is larger than 300MHz, there will exist high-mode. So how to understand the high-mode? Does the high-mode correspond to the different frequency?

Mode means the pattern or form or shape of the field. The fundamental mode is the mode that appears below the first cut-off frequency. Above the cut-off frequency the mode will differ.