What is the difference between Athan and Iqamah?

Athan is the call to prayer. Informing people that the beginning of the specific time (dhuhr or asr for example) has begun. I am not sure what Iqamah is because I heard calling from the mosque 20 minutes after athan then found out it is Iqamah. Is iqamah a second athan? Or is iqamah calling people to the mosque for congregational prayer? Does iqamah only apply to the mosque? (Sorry for not understanding. I'm still learning.)