In Octavia Butler's historical time-travel novel "Kindred," the most apparent significance of the title refers to—as the other two answers have stated—the familial relationships that Dana is called back in time to save. Dana, a black woman from the 1970s, is summoned back in time to save her ancestor, Rufus, a slave-owning white man, and ensure that he impregnates Alice, a free black woman who he later buys as his slave and concubine. The relationships Dana ends up forming with both Rufus and Alice are complex and difficult, especially because time passes differently between the two periods. Over the course of just a few weeks of 1970s time, Dana watches Rufus and Alice grow from small children into hardened adults. Try as she might to morally influence Rufus as he ages, Dana is no match for the social context of the antebellum South, and he turns into exactly the man she fears he will: a racist and abusive slave-owner who takes advantage of and hurts the people he loves. And try as she might to keep Alice hopeful of the possibility of freedom and escape, Alice ends up committing suicide, fulfilling her mother's belief that death is better than slavery.
But the idea of "kindred" can go beyond that direct familial lineage, and into all the ways that the individuals (and cultures) of the two time periods are similar, or related to, one another. Even though Dana is a blood relative of Rufus and Alice, they are still strangers to her when she meets them, as are all of the other people she meets when she goes back in time. The fact that she is able to form relationships with these strangers—close emotional friendships with other slaves, eventual moments of respect or understanding for Rufus's mother and father—shows the kinship of humanity that is revealed when living with and seeking to understand others. It is especially important that we see Dana extend this humanity to all the characters, black and white, in the face of the vast dehumanization that is slavery.
Dana is forced to reckon with the distance she had from her personal history (and from American history) by immersing herself in the past, and by experiencing the brutality of slavery firsthand. Her husband Kevin, a white man, who is accidentally brought back in time with her and is left there for five years, reckons with his place in history as a white man. While stuck in the past without Dana, he roams from place to place, helping slaves escape and trying to get by. So on top of the personal relationships the characters form with each other, there is also this disorienting sense of kinship with the past, which Dana and Kevin must reckon with in very different ways. Not only are there direct bloodlines connecting them to that era, there are direct cultural lineages connecting their 1970s life with that of the antebellum South. Dana and Kevin are coming of age in the time following the Civil Rights Movement. The historical kinship between slavery and the Civil Rights Movement is a glaring and easily tracked line. In the present time, Dana and Kevin faced animosity from both of their families when they announced they would be getting married. We see that the past could not be swept away that quickly.
The book is juggling and teasing apart all of these various interpretations of kinship, on individual and historical scales. The tenuousness of Dana's family line is reflected in the tenuousness of all relationships during that era—slave families separated by the whims of slave-owners, terrible medical standards, and the overwhelming violence that comes with such stark power differentials.