Stand out is the length that a metal blade tape measure can be extended horizontally before it ceases to be self supporting.
Stand out is directly related to the width and curve of the blade - typically the wider and more curved the blade is, the longer the stand out. The trade-off is that marking measurements from a heavily curved broad blade can be awkward and the tape will be bulky.
Stand out is valuable because it allows horizontal unsupported measurements to be made, i.e. the tape will not collapse and you won't need a second pair of hands for holding and marking out.