That is your O ring, that is worn out. You turn off your water of course from main valve, normally under sink, take apart faucet, top has set screw. take apart the part (lever the you use to turn on and off, under that is a o ring, there is also one at the base where facet hooks on at the base. What I have people do, to make it real easy for you to do yourself is, right down the model number, also take a picture of it, go down to your local home depot, or lowels or even a plumbing store, and show them the picture, as well as give them brand and model number. This is called the cheater way to fix. What they will do is show you the same facet or a brand that matches, and show you where the 0 rings are and how it comes apart and how to fix it, it is real easy job to do yourself. Instead of you reading a bunch of high tec manuals, this way it is hands on, sight on directions from a guy at a plumbing store. Don't feel to bad, cause you are going to buy the 0 ring from them. They usually cost like 5 bucks.
I hope this helped you , Mike
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