Jesus stands outside, knocking on a closed door with weeds. Holman Hunt's famous painting is full of symbolism, but the main idea, of Jesus knocking on a door, is taken straight from a passage in the book of Revelation...
"Listen! I am standing and knocking at your door. If you hear my voice and open the door, I will come in and we will eat together" (Revelation 3:20).
Holman Hunt later explained the meaning of the painting's symbolism: "The closed door was the obstinately shut mind; the weeds the cumber of daily neglect, the accumulated hindrance of sloth; the bat flitting about only in darkness was a natural symbol of ignorance..."
The picture was so important to Holman Hunt that he painted it three times, at different points in his life, and said that working on it the first time had made him become a Christian.