发句 means the beginning sentence (or sentences) in a poem (or article). 对句 means the middle part. 结句 means the last sentence(s) in a poem(or article).
好 means good.
可得 means could be attained, can be achieved, easy to get. 难得 means hard to get.
尤means especially.
So the whole sentence is literally, "Middle part good, easy to attain. Ending part, hard to achieve. Beginning sentence good, especially difficult to get," which means "It's easy to get a nice middle part, but it is hard to get a good ending, while it is especially difficult to get an impressive beginning."
Hello! I am so glad to write to you! I am from China and live in the Netherlands . I speak Chinese and English, and learning Dutch now. Looking forward to hearing from you and learn languages together:) All the best!