If I remember my physics classes correctly, this is a surprisingly easy phenomenon to understand.

1. Lasers heat things up. In this case, they're heating up air.

2. Hot air has lower-resistance than cool air. This is why lightning happens at all: as lightning goes through its path, it heats up the air hotter-and-hotter, which allows for more and more electricity to flow. That is, lightning itself heats up the air which makes the lightning more powerful.

3. This article talks about a device that uses lasers to heat up air ahead-of-time, where you heat up the air along pre-designated "safe" paths to provide a virtual lightning rod composed of this narrow stream of hot air... rather than a metal rod.