Topology is the branch of mathematics (geometry, to be more precise) that deals with the quantification, study, and manipulation of surfaces, spaces, and the deformation thereof.
Put another way, topology is the study of how surfaces and spaces can be manipulated and deformed while maintaining their intrinsic properties, and what differentiates various spaces from one another.
For instance, in topology, a sphere and a box are identical because their surface points are all "connected" to each other in the same way. You can deform a sphere into a box.
However, a sphere and a donut are different. You cannot deform a sphere into a donut without tearing or joining surfaces, changing the way the surface points are connected.
Likewise, a donut can be deformed into a coffee cup, but not into a kettle.
Therefore, the science of topology is concerned with the characteristics of spaces and surfaces that define and differentiate the different types of spaces and surfaces. For instance, if the actual set of points in a 3-D space that define a sphere is not necessary for defining the type of topology that the sphere is, then what does mathematically differentiate a sphere/box from a donut/coffee cup or kettle?
Talking about the topology of space is a discussion about the "shape" of space in its 3-D projection. Is space closed and spherical/oblong/rectangular/toroidal? Is space flat? Is space saddle-shaped and hyperbolic like a saddle?
Whatever the shape of the 3-D projection of space - even if space is perfectly flat - local geometries are curved by the presence of mass and energy.