Using the word "separation" in the context of marriage can mean a couple of different things.
On an emotional or even existential level, the two parties may feel distant from each other - even if they live in the same house. In this sense, there can be a form of separation involving couples who are still married.
If each party lives in a different location, that marks another type of separation. This does not necessarily mean, however, that the couple is on the verge of divorce. The partners may live in different cities, for example, due to job market reasons - not because they aren't connected.
But there is also a more formal status that is called legal separation. To be sure, this status lacks the finality of a divorce. But when legal separation has become formal, it is possible to convert it to a full-fledged divorce at a later time.