The Talmudic explanation is that a concubine is a wife without a written marriage contract. In a modern context, that makes "concubine" equivalent to "de-facto" or "common law" wife. So, a very common and normal situation even today, not something strange and novel.
This is consistent with the idea that a concubine is a "lesser status" wife, because things like inheritance are contractual arrangements which are either directly addressed in or influenced by a written marriage contract. A de-facto wife does not have the benefit of any such formalities, so depending on the legal environment has lesser legal standing than a formal, documented wife.
It is very dangerous to get too enthusiastic about concubines and think they are something different and exciting, something we could have today for extra sexual pleasure for instance. Too many men have been caught in this trap. As the word "concubine" is not defined in scripture itself, and most people are either unaware of the Talmudic definition or would understandably reject anything from the Talmud anyway (I'm uncomfortable using it to define the word myself, only using it as scripture does not give us a definition and I am not willing to just invent one), it is possible to redefine "concubine" to mean almost anything you want it to mean. So men can behave in ways that most of us would condemn as sinful, but label it "concubinage" and convince themselves it is ok, making it right in their own eyes.
This is why
@The Revolting Man has always been so against promotion of concubines - and he's right to be concerned. We have seen this used to excuse sin and ruin people's lives.
Here be dragons.
But there is no danger whatsoever if we simply see "concubine" as a synonym for de-facto / common-law wife, because that is a relationship we all accept as valid. It is also completely consistent with the usage of the word every single place it occurs in scripture. It is a safe, conservative definition that does not risk justifying error. Given the uncertainty around the word, assume the most conservative definition that does not risk leading you into sin if you happen to be wrong.