If the chip is the kind that you wear on a Velcro strap round your ankle, or attach to the laces of your shoes, you have to remember to give it back at the end (there are generally pretty obvious people at the finish collecting them), or generally you'll be charged a fee (usually about ??10). If it's the kind that's attached to your number they're disposable. Don't worry, it will all be explained in the pre-race info.