"The second lesson is very easy."
Translation:La deuxième leçon est très facile.
The fundamental difference between “deuxième” and “second(e)” is not set in stone, but, basically:
deuxième is generally used when the list will have more than 2 (or 3) items.
second (feminine form: seconde) is generally used when it is the end of the list. (but sometimes there can be a third item too).
As we don't have enough information to know if the lessons continue (Eg une cinquième leçon) or stop at two (la seconde leçon), both are accepted (in this instance they are synonyms).