Being overweight increases the risk of a number of serious diseases and health conditions. Below is a list of said risks, according to the Centers for Disease Control and Prevention (CDC):

High blood pressure
Higher levels of LDL cholesterol, which is widely considered "bad cholesterol," lower levels of HDL cholesterol, considered to be good cholesterol in moderation, and high levels of triglycerides
Type II diabetes
Coronary heart disease
Stroke
Gallbladder disease
Osteoarthritis, a type of joint disease caused by breakdown of joint cartilage
Sleep apnea and breathing problems
Certain cancers (endometrial, breast, colon, kidney, gallbladder, liver)
Low quality of life
Mental illnesses such as clinical depression, anxiety, and others
Body pains and difficulty with certain physical functions


Generally, an increased risk of mortality compared to those with a healthy BMI
As can be seen from the list above, there are numerous negative, in some cases fatal, outcomes that may result from being overweight. Generally, a person should try to maintain a BMI below 25 kg/m2, but ideally should consult their doctor to determine whether or not they need to make any changes to their lifestyle in order to be healthier.