It implies that reading is like watching water, and it is endless. Comparing books to water tells us that we should constantly replenish new knowledge, learn and enrich ourselves in order to reach a new realm.
The source is Zhu, a scholar in the Song Dynasty, who wrote two reading poems.
Reading Two Thoughts is a set of poems written by Zhu, a scholar in Song Dynasty. These two poems describe his feelings of "reading" and reveal profound philosophy with vivid images. The first poem refers to the phenomenon of fresh water pouring into the pond, which means that we must constantly accept new things in order to keep our thoughts active and progressive. The second poem can sail freely in the water with the help of a huge ship, which shows that artistic creation needs inspiration. The whole poem contains philosophy with vivid metaphors and is full of rational interest, which has been told all the time.
References:
Baidu Encyclopedia: Two Thoughts on Reading