A deaconess is someone who is like a helper in a religious group, kind of like how Mom or Dad helps you at home. They usually help take care of the church and the people who go there.
Like when you have a boo-boo, your mom or dad helps clean it up, give you a band-aid, or help you feel better, right? Deaconesses do some of the same things, but they do it at church. They might help people who are sick or need some extra help, like cooking food or running errands.
Deaconesses might also help with things like baptisms, which is when someone gets a bit of water on their head to show they love and follow God; or weddings, which is when two people who really love each other decide to get married in front of their friends and family.
So basically, deaconesses are like special helpers in a church who make sure everyone is taking care of and happy.