The diagram uses black Xs to show the vector representation, in a 2D space, of word embeddings. The red Xs correspond to the resulting sum vectors S1 and S2 of each sentence (S1 representing the sum vector of the sentence "The astronomer observed the stars" and S2 representing the sum vector of the sentence "The stargazer watched the celestial bodies"). The relative proximity (in terms of, for example, cosine distance) of the sum vectors shows that the meaning of the two sentences is very similar.