The metronome is a nifty gadget that moves and makes a sound at a regular rate. This provides a musician with a visual and aural aid to enhance their own sense of rhythm. Beethoven was the first major composer to embrace the metronome.
All lead guitarists need to practice using a metronome. No matter how good you think you sound or how great your sense of rhythm, the metronome and the cassette recorder or cellphone or whatever recording device you use will show up the flaws in your natural abilities.
Once you have established that your playing is not entirely perfect, how do you begin to work on it? By setting the metronome way below your comfortable level of playing. You need to practice really, really slooooow! You need to teach your muscles to execute your guitar playing in a calm, relaxed manner without unnecessary tension. If you force youself to practice like this it will be uncomfortable at first but it will become second nature.
More info on muscle memory for guitar players is here.