The two main words to describe something that requires a lot of effort to deal with or to understand are schwer and schwierig. Although they are used almost interchangeably, schwer also has the meaning of “heavy”.
Likewise, the opposite, “easy” is either leicht or einfach, where leicht also means “light” (in the sense of not weighing much).
Examples
Das ist zu schwer für mich. That’s too difficult/heavy for me.
Das ist zu schwierig für mich. That’s too difficult for me.
Es ist leicht den schweren Tisch zu bewegen. It’s easy to move the heavy table.
Die Hausaufgaben sind einfach. The homework is easy.
Dieses Buch ist sehr leicht. This book is very easy/light.
Dieses Buch ist sehr leicht zu lesen. This book is very easy to read.
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Good to Know
As an interesting aside, the word schwer is related to the English word “serious”, which again comes from Latin and means “weighty” or “important”.
Also interesting is the history of the word “tough”, which means “difficult to chew” and as such is related to the German word zäh with the same narrow meaning. (Source: http://www.etymonline.com/)