甘甜 is a literary and evocative way to say 'sweet', stronger and more poetic than simply 甜 (tián). It is often used to describe the taste of water, fruit, or air, and figuratively for pleasant emotions or memories.
memory
Both characters mean 'sweet': 甘 (gān) = sweet/pleasant, 甜 (tián) = sweet. The doubling intensifies the pleasantness.
