Ok please please please someone answer this with a little detail...just a paragraph! Please answer as soon as!?

You need the region to which the primer is binding to be conserved throughout all of the template DNA. If there are any changes in the template at the site of the primer, the primer may not bind as selectively, and you will not get a pure product. You would either get a mix of many different pieces of DNA (if your primer was smaller and involved in a lot of nonspecific binding) or no DNA product.