"Trilaminar emdometrium" is a term generally applied to the (ultra)sonographic pattern of the endometrium. It speaks to the "shape" of the interuterine area and, by default the echoic properties of the endometrium, which is the lining of the uterus. Let's back up.
The uterine lining, the endometrium, undergoes changes throughout a woman's menstrual cycle. In cases where a woman is undergoing some sort of fertility treatment, sonography reveals something about the characteristics of the endometrium, and investigators may suspect that conditions may or may not be favorable to fertilization and conception based on what they see in the sonogram. A trilaminar endometrium is more favorable than non-trilaminar.
What seems the case is that if the "general shape" of the image is a trilaminar endometrium, then the conditions for favorable outcomes in insemination attempts are improved over non-trilaminar sonography. There is not guarantee, but in the clearly non-trilaminar imaging, no pregnancies occurred in the studies.