The concept is to reduce the size of the spacecraft to about the size of a chip used in electronic devices. The idea is to launch a thousand of these mini-spacecraft into the Earth’s orbit. Each would have a solar sail. This is like a sail on a boat – but it is pushed along by light rather than the wind. A giant laser on Earth would give each one a powerful push, sending them on their way to reaching 20% of the speed of light.
Source: Hawking backs interstellar travel project – BBC News