得 can be used as a complement that helps indicate completion or consequence of the action (preceding verb). Its use is traceable to classical Chinese (see 10th entry here):
By thirteen, an accomplished lute player I had become; In the imperial registry of the first section my name be found. (translation)
You master something if you ...
不安 mean uneasy, restless. The close opposite is 鎮定, which means a person with a stable mental state when facing either expected or unexpected events (can be any mind-stimulating event but usually bad things or news).
The complete opposite is 安然, which means at ease.
verbal particle used to express potentiality or to indicate a result or an extent
'得'(can/ able to) in '聽得到'(can hear) is a [potential particle]. The other potential particle is '不'(cannot/ unable to) as in '聽不到'(cannot hear) -- there are only two potential particles
'得' (to the point of/ result in) in '聽得入神' (listen to the point of being ...