What does Ukemi mean to you?
Why is it important in your practice?
What does it feel like?
When has it helped you?
Also any links to helpful articles or books would be greatly appreciated.
To my mind ukemi refers to a process of receiving; in that sense it's also about how to apply yourself. So from my limited understanding it's tied deeply to how to throw/pin/etc. safely.
In my practice its importance comes from the beneficial effect on my body and mind. My body feels more supple after receiving gently applied techniques (especially when I'm engaging more or less well), and less sore on average when less gently applied techniques are done on me, and my attention/awareness is focused when I actively consider how to not get hurt; how to safely apply effort while effort is being applied to me.
It feels somewhat different based on the different people I've trained with and how well I'm focusing and engaging myself.