Suppose you agree to rent an apartment. The lease agreement you sign with the landlord is the main contract. However, your landlord promises to fix the toilet drainage. Therefore, this is the collateral contract.
Examples of collateral documents are a security agreement, guarantee and collateral agreement, pledge agreement, deposit account control agreement, securities account control agreement, mortgage, and UCC-1s.
A security agreement is not used to transfer any interest in real property (land/real estate), only personal property. The document used by lenders to obtain a lien on real property is a mortgage or deed of trust.