The experimental probability of a certain event is the proability for that event to happen, "suggested" by some empiric experiments, instead of pure theory.
For example, we know that a fair coin has a 50% chance of landing on heads or tails. But if you throw a coin 100 times, with 60 heads and 40 tails, you would say that the probability for heads to show is 60%.
In your exercise, Jenny tried the experiment 50 times, and got a pink tile 18 times. So, according to her results, the probability of pulling a pink tile is