Role of equity

[Mark Forest]

In another thread I saw the following written:

 Generally, if you buy a junior lien, you want to make sure there is enough equity in the property to cover the 1st lienholder, that way, if the property sells, it now has a better chance to cover 1st & 2nd lien.

Is he talking about the original owner's equity?  Why is that relevant?  I have heard this type of comment before.  I admit I am still learning about sheriff auctions.