Memory Card Slot
Memory card slots are used primarily to add storage memory to a phone, in the form of a memory card. This extra memory can be used to store and transfer photos, videos, music, computer files, or backups of phone data such as the phone book and calendar.
There are several different sizes and types of cards. The most common type of memory card in phones - by far - is microSD (as of 2017):
Adaptors are available for different sizes of SD cards. For example, a microSD card can be used in a full-size SD card slot with the proper adaptor.