"O czym jest ta lekcja?"

Translation: What is this lesson about?

[-Relentless-]

I put "what is this lecture about?" why is that wrong? Isn't a lesson a lecture?

[alik1989]

As strange as it sounds, but "lecture" is more commonly translated as "wykład", whereas "lekcja" is a lesson, a class or a course.

[SofiiaBlav]

Lecture is a better version of lekcja, not lesson

[alik1989]

No it isn't, and this has already been discussed in this thread.

[TomNies]

Literally "about what is this lesson".