A Study to Assess the Safety of BIIB122 Tablets and if it Can Slow the Worsening of Early-Stage Parkinson's Disease in Participants Between the Ages of 30 and 80

In this study, researchers will learn more about a study drug called BIIB122 in participants with early-stage Parkinson\'s disease (PD). In this study: * Participants will take 225 milligrams (mg) of BIIB122 or a placebo as tablets by mouth. A placebo looks like the study drug but has no real medicine in it. * Participants will take BIIB122 or placebo 1 time a day for up to a minimum of 48 weeks and a maximum of 144 weeks. * Certain medications for PD will be allowed at enrollment for a subset of participants. * Participants will have to visit at 2-week intervals between baseline and week 12 and at 4-week intervals between week 12 and week 48 and at 12 week intervals between week 48 and week 144. The main question researchers are trying to answer is if taking BIIB122 slows the worsening of symptoms more than placebo in the early stages of PD. To help answer this question, researchers will use a questionnaire called the Movement Disorder Society-Unified Parkinson\'s Disease Rating Scale, also known as the MDS-UPDRS. Researchers will use the MDS-UPDRS to learn about participant PD symptoms and how they affect their daily life. Researchers will also learn more about the safety of BIIB122.