The video above has some helpful tips for designing PCR primers for one purpose - cloning with restriction enzymes. PCR or Polymerase Chain Reaction, is a very common tool used in molecular biology to amplify fragments of genetic material. In some cases, that material might then be inserted or cloned into a vector, another common molecular tool. Vectors come in many variations and can be used for a variety of purposes, and it's easy to manipulate the genetic sequence of interest once it's been cloned into a vector.
Restriction enzymes, yet another tool in the molecular kit, make cuts at specific, short genetic sequences. Restriction enzyme sequences can be added onto the ends of a genetic sequence through PCR, and they have been conveniently packed into vectors so genes can be easily inserted by using them. When using them to clone some DNA into a vector, the restriction enzymes have to be chosen carefully. Of course, it's important that they not cut the sequence of interest. They also should work well together. Catch a few more pointers from the short video.