Verisimilitude is the believability of a fictional work. Does the performance and/or writing convince you that the action was possible even though you know it was fictional?
Veri means truth. Simili means likeness. Tude is state of being. Having verisimilitude is having the appearance of truth.
The quality of appearing to be real. Stage sets are not real, but they can have an astonishing amount of verisimilitude.
Versimilitude, or little details that are actuallytrue comes a long way in making a story realistic. These do not need to be major details, things such as a place name, or a minor person with little effect on the story can make all the difference. It is not about making the fiction realistic so much as it is about making the world the fiction takes place in seem realistic. If the universe sounds like it might exist, the reader is more willing to go with the author. But get the little things right, such as places, names, historic events, and the reader will believe the world exists. This can also be taken further out into tricking the reader's mind. For example if one were to right a story set on a alien world in a different year, one can not have historical details to add realism to it, but what one can do is to create those events, they need not be drawn out, but mentioned in passing. These little details make the universe seem real. And the last bit of advice is on the characters. Make them partially flawed, any ubermensch will subtract substantially from the reality, instead ensure each character has a flaw, but do not make the flaws obvious, because a character that is "made of fail" as it were is just as unbeleivable as a perfect character. Take for example character A who is a glutton, the appropriate way to show realism would be to show the character eating maybe a bit more than the other characters each time. Let the reader's mind connect the dots, do not tell them.