I'm sure this is a very simplistic take, but from what I've read this is an ongoing struggle between the Tutsi and the Hutu peoples. Colonial powers used the Tutsi to suppress the larger Hutu population. This created societal divisions and hatred that still exists to this day and manifests in different violent incidents in many countries in the area.
Rwanda wants to take over the Congolese region that borders it, using this wider racial conflict as an excuse. There's been (mostly guerilla) battles taking place in that area for close to 3 decades now. Recently, some Congolese villages and towns were handed over to Rwandan forces by elements of the Congolese army, though the Congolese say this "hand-over" was staged after Rwandan-backed rebels actually took over these towns, and now the Congolese army is fighting to to take those back.
I don't know enough to judge what exactly is true and isn't true. It seems that the UN chronically considers Rwanda to be the aggressor in this conflict.