Hello,
if tumor is malignant, it is yes then y=1, if no 0 that does not define the threshold.
threshold is the ability to find as small as possible defect, tumor, abnormality in a data, so as low as you can find, it is better.
please refer this Week 3 decision boundry sigmoid function - #2 by Deepti_Prasad
Hope you understood!!
Keep Learning!!!
Cheers
DP