The acronym stands for Adult Baby / Diaper Lover. But the name doesn’t fully describe the variety of interests that fall under the “ABDL” label. Some people enjoy the feeling of being treated like a small child and wish to wear diapers while being taken care of by an affectionate partner. For others, diapers are a part of a sexual interest called Paraphilic Infantilism which may or may not involve an interest in role-playing. Some individuals wear diapers for medical or practical reasons and belong to the ABDL community for support and access to the large amount of information and products available to ABDLs.
There’s great variety between people’s individual interests and the broader ABDL community supports this diversity. You’ll find different definitions and explanations everywhere you go. If you are looking for more information, I recommend perusing Understanding Infantilism which is the classic resource on this subject. The clearly written articles are backed by research and cover many different viewpoints.