What does this seal stamps on chinese painting mean?
芳草有情，夕陽無語，雁橫南浦，人倚西樓。


This sentence is a part of 宋詞 (a form of classical Chinese poetry
)《風流子·木葉亭皋下》 written by 張耒.

張耒（1054年－1114年），字文潛，號柯山，生於楚州淮陰（今江蘇淮安市），祖籍亳州譙縣（今安徽亳縣）。北宋詩人。

The whole article is shown below.


  木葉亭皋下，重陽近，又是搗衣秋。
  奈 愁入庾腸，老侵潘鬢，謾簪黃菊，花也應羞。
  楚天晚，白蘋煙盡處，紅蓼水邊頭。* 芳草有情，夕陽無語，雁橫南浦，人倚西樓。*  
  
  玉容知安否？香箋共錦字，兩處悠悠。
  空恨碧雲離合，青鳥沉浮。向風前懊惱，芳心一點，寸眉兩葉，禁甚閑愁？
  情到不堪言處，分付東流。


This poem describes the feeling of yearning for his wife.

However, the maker of the seal has changed one word, i.e., 橫 to 飛。

雁橫南浦 --> 雁飛南浦

The meaning is the same.