Iodine Clock Reaction

How does the concentration of iodide ion change during the initial portion of the reaction, while there is still unreacted thiosulfate present

http://quiz2.chem.arizona.edu/preproom/Demo%20Files/the_iodine_clock_reaction.htm

how does this website answer the question? I don't see anything about the concentration.

You did not read it carefully. Reading critically takes time: inference skills are needed. Look at the two reactions: One slowly converts the iodide ion into iodine, which is instantly coverted back to the iodide ion. Does this mean the iodide ion concentration is
a) constant?
b) reduced?
c) increased?
If you had 100 basketballs that slowly changed to bananas, but when changed, instantly changed back to basketballs, how many basketballs could you count at any one time?
Use your reading skills here to figure it out. Now whent he the tiosulfate in isused up, then the instant conversion stops, and the iodine ion is slowly converted to iodine.

so does that mean the concentration is constant?

very nearly constant until the tiosulfate is used up.