Question

What is placebo and why is it used in some experiments

Answer 1

A placebo is basically an alternative medical treatment (e.g. pill)  that doesn't contain an active substance meant to affect health. So it isn't actually solving anything but it can have the placebo effect. Which can be assumed is the reason it is sometimes used in experiments. 

Answer 2

A placebo is kind of a "fake" treatment but, typically the person receiving a placebo is unaware it has no medical effect on them so they believe they are feeling better. For example, a lot of anxiety meds are placebos and people don't know so their brain tricks them into thinking they are feeling less anxious. This could be used in experiments where a group believes they are affected to see how they react