Type uniqueidentifier is primary key and foreign key also

Hi Everyone
Step 1: I  have a store procedure (sp1) -> with data.

 Step 2:  I created a store procedure, with Column TransactionID is the primary key and foreign key.

But I don't understand , Cloumn TransactionID is the primary key and foreign key.  I used NewID() in Store Proceduce.
I do not know how to handle. Expect people to help.
Thank ...