The L'Tar Nebula was an interstellar gas cloud located near Breen space.
In 3191, Ambassador Saru informed Primarch Tahal that the Federation was willing to establish an exclusive trade route through the L'Tar Nebula to her border, which would bolster her claim to the throne of the Breen Imperium, in exchange for her stopping her attempt to seize Ruhn's dreadnaught. Tahal rejected the proposal immediately, which confirmed to Saru Federation Intelligence rumors that she had bases inside the nebula.
In a subsequent communication, Saru claimed to Tahal that many planets in the region were prepared to assault her bases inside the nebula, unless he ordered them to stand down. Tahal accused him of bluffing, but ultimately relented and turned her fleet back. (DIS: "Life, Itself")