The spark plugs fire in the cylinder to ignite the fuel creating combustion. The replacement of these should take place every year to 2 years depending on what you drive. They are very inexpensive parts and optimize your cars fuel efficiency. Its very recommended that you have these changed consistently with your regular maintenance when they are ready. Most mechanics can pull a plug and tell you if you are ready. If you pull them yourself, look for rounded electrodes or rust around the porcelain. Also if you find oil on the plug, your problems just got much bigger and you definitely need to bring it in.