As the second half of an ABB adjective it makes a feeling light and gentle: 甜丝丝 'pleasantly sweet', 凉丝丝 'a little cool', 冷丝丝 'a bit chilly', 暖丝丝 'gently warm'. These words usually end in 的.
memory
丝 is a silk thread. Doubled, it pictures something arriving strand by strand, each as fine as silk.