Hi Yuan,
What is displayed are the significant t-values of your statistics test, with values that have p-values higher than the significance level threshold (α) set to zero (this is adjusted in the Stat tab). Note that your colormap seems to be in absolute value here as the regions you have selected have a negative t-value yet it is displayed in yellow.
For more information, please refer to our statistics tutorial:
https://neuroimage.usc.edu/brainstorm/Tutorials/Statistics
Best,
Martin