From TCL's link:
"UMPIRES CALL is a way of saying decision made by the on field umpire should stand
The rules of the referral system say that there needs to be a clear mistake by the on-field umpire to reverse the decision. "Umpires Call" is a way of saying that there isn't a CLEAR mistake, and therefore the decision should stand.
What’s right or wrong?
High tolerance given to umpires: As inferred from the rules above, the tolerance given to the on-field umpire is equivalent to the extent of an additional half of the cricket ball on either side of the outer stumps, when it comes to the ball impacting and hitting the wickets within the demarcated zone."