Let us now set aside the question of the submicroscopic structure of space-time and concentrate, instead, on its large-scale properties. In this case, we may imagine that the smooth manifold picture will be adequate and that its structure in the large can be obtained by piecing together smaller "locally Euclidean" patches, in the manner of overlapping coordinate neighborhoods of differential Geometry. Thus we might arrive at a topology for space-time, in the large, different from a Euclidean topology. Unfortunately too little is known about the large-scale structure of the universe to enable us to make any statement with confidence concerning its global topology (apart, perhaps, from certain statements about its orientability). Thus, it might be that the topology of space-time on a large scale is not at all interesting.

Battelle Recontres, Sir Roger Penrose, page-123.

Could it be explained what exactly Penrose means when he says the topology of space-time? How can one picture it? I am assuming when he says Euclidean topology, he is talking about the metric ball topology in $\mathbb{R^4}$.