Quoting from the Huffington Post article:

...“dimming” involves deliberately minimizing a hotel’s appearance or ranking in an online agency’s results.

It’s the byproduct of a behind-the-scenes conflict between hotels, which want customers... to book directly with them, and online travel agencies, which don’t want to be undercut by the hotels. The bottom line for customers: When you book online, you may not see the cheapest hotels first. In extreme cases, you may not even be able to book the hotel you want on the agency’s site.

The dimming problem flickered to life this spring, after hotels won a series of court victories in Europe that effectively allowed them to offer lower rates on their own websites, according to Dori Stein, the chief executive of Fornova, a technology company that works with hotels. Previously, hotels had contracts with online agencies that gave the agencies’ sites their best rates.

“Online travel agencies retaliated by dimming,” Stein said.