1. FODMAPs are carbohydrates that some people's bodies can't digest very well.
2. If your body can't absorb FODMAPs correctly it can cause gas, bloating, cramping, and/or diarrhea.
3. A low-FODMAP diet is therefore used to treat people with symptoms of IBS or similar to IBS. But you should not try a low-FODMAP diet without consulting a doctor because that might lead to nutrient deficiency.
This is in no way a complete list of foods that should be avoided or embraced. Many helpful lists available online differ slightly, which is why speaking with a doctor or dietician is important. Everyone's elimination diet will work differently, as will the reintroduction period.