A seam is two separate pieces of fabric stitched together.  Seams can be curvy or straight and you'll want to practice so that your seams are even, because that'll affect the outcome of how professional your project will look in the end.  Many of the projects you will do will require a lot of seams in order to bring the whole project together.  You'll be amazed at how just doing a simple seam can turn bits of fabric into something amazing and something that you'll be proud of.  And it all started with something as simple as a seam.