Let's imagine that you want to photocopy a volume of encyclopedias. That's a lot of information/pages, right? This is similar to copying eukaryotic DNA. Loooooots of information to copy.
If you have, say several friends who can help you photocopy, how will each friend know which pages to begin at and which pages to end at? This is similar to the issue with DNA polymerase III. How does it know where to begin and end copying a section of DNA?
Let's say that you decide on using Post-It Notes to mark where one person begins and the other person will end. This is similar to the function of the RNA primer.
DNA polymerase can only "see" 3' ends. If there's no 3' end, it's basically blind! It has no idea where to begin! The RNA primer has a 3' end that tells the DNA polymerase "Hey - start here!" The DNA polymerase III will then start adding DNA nucleotides to the 3' end and will stop when it no longer "sees" a 3' end.