Can you explain: "Soon after we can see, we are aware that we can be seen. The eye of the other combines with our own eye to make it fully credible that we are a part of the visible world. If we accept that we can see that hill over there, we propose that from that hill we can be seen."

This quote is part of an argument that John Berger is building about oil painting and modern advertising. He contends that advertising is the heir of oil painting: it functions in a similar but not—and this is important—identical way.
Both oil painting and advertising are about seeing and being seen, which is why Berger talks about both in this quote. In oil painting, the wealthy and their grand material goods are seen. The poor are largely invisible. An oil portrait or landscape proclaims the owner's power and privilege and puts it on display for others to gaze at. It helps reinforce the social order: some are higher and some are lower. The unseen lower orders looking at a grand oil painting depicting wealth would not, however, envy it. They would merely be awed and recognize the higher-class person's greater status.
Modern advertising also relies on the people depicted in it being seen, as in earlier oil painting. However, Berger argues, advertising exists to create envy; it implies that we too can be the "seen" rather than unseen and become desirable as well if we buy and use the products advertised.