Portfolio- to put it simply this is a visual resume for a model. A modeling portfolio is a collection of printed test images, tear sheets, and other print/digital work that a model has done. Now and days most models carry there portfolio on a iPad or Tablet, having a physical bound printed portfolio is recommend though.
Pro-tip 1: You do not need to have a portfolio before you can be signed with a agency. This is a common myth, all you need are digitals.
Pro-tip 2: The common size of a portfolio book is 9.5 x 12 or so, 8.5 X 11.5 works too!