The "winter" was characterized by reduced funding and interest in the topic. The term was coined by analogy to a nuclear winter.
I think meme researchers have experienced some winters of their own. Probably the main winter was from around 1900 to 1970 - when interest in the topic began to increase in a number of areas.
There was a large and obvious meme renaissiance in 2011. Around the same time, cultural evolution became much more popular, with many more researchers and papers appearing. However, memetics itself has been slow to experience the knock-on benefits of this. Indeed, compared to the situation at the turn of the century, interest in memetics seems to be at a low ebb.
Essentially, I blame academia for this situation. Academia is sensitive about the issues surrounding social Darwinism. A "tread softy" approach appears to have been taken. Notoriously, academia is little concerned with finding the truth, but rather is about with credentialing experts and forging affiliations. The meme pioneers included a lapsed parapsychologist and a self-help-guru and poker player - and were a raggedy bunch. For whatever reason, the outcome has been a disaster. Most researchers within academia seem ignorant or confused about memetics - and are often ignorant or confused about the whole idea of cultural evolution too. It's an ongoing disaster for scholarship in the area.