Q:

What does Focault mean by “space”?

I’m reading “Of other spaces”, but it’s hard to follow for me because I don’t know what Focault means by “space”. So, what does Focault mean by space? What would be the definition?

A:

IMO, "space" refers to space... what can be more difficult to understand, is the use of the concept made by Foucault.
See : Christopher Falzon etc. (editors), A Companion to Foucault, Wiley-Blackwell (2013), Ch.19 Space, Territory, Geography by J.W.Crampton :

“Space” is a polyvalent term that until the early twentieth century could be read simply as objective absolute space within which “objects exist and events occur”. Foucault rarely treats space this way, preferring to treat it as an element of power, discipline, or governmentality. [...] In sum, for Foucault space is not a pre-existing terrain. Rather, in his work it is the very “production of space” and its relation to power that is at stake.

And see : Dianna Taylor, Michel Foucault: Key Concepts, Rouledge (2014), page 29 :

Foucault insists that disciplinary power creates a cellular form of individuality by ordering individuals in space. He calls this ordering
  "the art of distributions" . Cellular individuality rests on the division of
  individuals from others. The art of distributions produces this individuality
  by first of all enclosing a space different from all others through the
  use of walls or gates, as in the case of barracks and factories (Foucault
  1979). It partitions this space into individual cells in order to
  break up collective activities that deter from the goal of utility, such as
  desertion or vagabondage . The art of distributions also codes a space
  with specific functions to make it as useful as possible.

See Of Other Spaces: Utopias and Heterotopias (1967) :

Yet it is necessary to notice that the space which today appears to form
  the horizon of our concerns, our theory, our systems, is not an innovation; space itself has a history in Western experience, and it is not possible to disregard the fatal intersection of time with space. One could say, by way of retracing this history of space very roughly, that in the Middle Ages there was a hierarchic ensemble of places: sacred places and profane places: protected places and open, exposed places: urban places and rural places (all these concern the real life of men). In cosmological theory, there were the supercelestial places as opposed to the celestial, and the celestial place was in its turn opposed to the terrestrial place. There were places where things had been put because they had been violently displaced, and then on the contrary places where things found their natural ground and stability. It was this complete hierarchy, this opposition, this intersection of places that constituted what could very roughly be called medieval space: the space of emplacement.
This space of emplacement ["espace de localization"]was opened up by Galileo. For the real scandal of Galileo’s work lay not so much in his discovery, or rediscovery, that the earth revolved around the sun, but in his constitution of an infinite, and infinitely open space.

It is clear that Foucault reads the transformation from the closed, hierarchical cosmos of Ancient and Medieval worlds to the infinite universe of Early Modern times not only from the point of view of natural sciences, but also in broadly social and political terms.

Emplacement means, according to Foucault, that relations between locations in space are the constitutive principle of space perception. [...] We still divide to inner form the outer, the internal from the external and assign different meanings to different types of spaces depending on their mutual relations.

