This study is open to adults 18 and older with an eye condition called diabetic macular edema. People are required to have a specific type of diabetic macular edema called centre-involved diabetic macular edema (CI-DME) to take part. The purpose of this study is to find out whether a medicine called BI 1815368 improves sight in people with CI-DME and to find the most suitable dose. This study has 2 parts. In the first part, participants are put into 2 groups of equal size randomly, which means by chance. One group takes BI 1815368 tablets and the other group takes placebo tablets. Placebo tablets look like BI 1815368 tablets but do not contain any medicine. In the second part, participants are put into 4 groups of equal size randomly. 3 groups take different daily doses of the study medicine, BI 1815368, while 1 group takes placebo. All participants take tablets twice a day for about 11 months. Participants are in the study for about 1 year. During this time, they visit the study site 16 times. At visits, doctors check the participant's vision and collect information on any health problems. They take detailed pictures of the eye. The changes over time are compared between the groups to see if the treatment works.