U.S.residential mortgage backed securities (RMBS) backed by second-lien collateral中的second-lien collateral是什么意思?

backed by second-lien collateral
我想应该是不属于自己的财产证明,指的其他的亲属的.
collateral
.并行的
2.附属的,附带的
digress into collateral matters
扯到枝节问题上去
3.旁系的;间接的
A cousin is a collateral relative.
表兄弟是旁系亲属.
4.担保的,有担保的
a collateral loan
抵押贷款
n.
1.旁系亲属[C]
2.担保品,抵押品[U][S1]