A half-life is the amount of time it takes for one-half (or 50%) of the original amount of material to be changed to a different element. Here's what this looks like graphically:
After 10 half-lives, only 0.1% (or 1/1,000th) of the original amount of material is left. The software cannot draw the line thin enough to truly represent the fraction remaining.
Iodine-131 (131I) has an eight-day half-life. After eight days, half of the 131I is gone. After 16 days, half of that is gone. Let's look at this graphically, assuming we have 8,000 Bq at the start. After one half-life, we have 50% of the original amount left, or 4,000 Bq.
After 10 half-lives, only about 1/1,000th of the 131I remains.