Traditionally it is good practice in sewing to pin, tack and then sew. For beginners, a line of tacking stitches can give you a guide for your machine sewing. Although pins are good, takcing means you do not have to keep starting and stopping to remove pins while also holding your fabric layers together, preventing the fabric from sliding out of place when sewing.
 
Tacking stitches are temporary stitches and tacking thread is softer than standard sewing thread so it can easily be removed from your fabric, without damaging your fabric or project. 
 
A soft, lightweight 100% cotton thread.