-는 is the present tense action form of 듣다 (to hear, listen, take a class).
듣는 수업 means "the class you're taking".
A: 요즘 듣는 수업이 어때요? = How is the class you're taking these days?
B: 처음 에는 어려워서 좀 힘들었는에 이제는 [쉬워졌어요] = I had difficulties in the beginning because it was tough, but it got easier now.
(쉽다's stem becomes 쉬우 when it comes before a vowel (ㅂ-irregular conjugation) = 쉬워, 쉬우니까, 쉬운)
One tricky thing with this type of expression is that Korean often drops the subject and marker particles, making them look ambiguous sometimes.
- 생선을 먹는 고양이 = a cat eating fish (고양이 is the subject of 먹다)
- 먹는 것 = things to eat (것 is the object of 먹다. The subject is people and animals in general)
- 듣는 수업 = (당신이) 듣는/듣고 있는 수업 (수업 is the object, subject is omitted)
The noun after -는 can be in different relations to the verb, so that the contextual knowledge plays an important role.
While learning, this might look unclear compared with English where the subject is usually there to make it more clear. But it makes it more flexible and versatile too. You need to be familiar with how things are customarily left out to understand such phrases.
As for "listening class" (i.e. a class about listening), we will usually say 듣기 수업 where 듣기 is the noun corresponding to "listening".