What is this "self soothing" people speak of???

Kyle
My baby has fallen asleep twice on her own, ever. And both times in her stroller with the movement.
Other babies lay down in their cribs and fall asleep on their own? That's actually a thing? Am I doing something wrong? 
Ayla only falls asleep on the breast or being bounced to sleep in our arms. What are ways to start getting her to sleep on her own? (Not into CIO). 
Thanks in advance for any tips! 

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