The Ice Bucket Challenge, sometimes called the ALS Ice Bucket Challenge, is an activity involving dumping a bucket of ice water on someone's head to promote awareness of the disease amyotrophic lateral sclerosis (ALS) and encourage donations to research. It went viral on social media during July-August 2014. In the US, many people participate for the ALS Association, and in the UK, many people participate for the Motor Neurone Disease Association, although some individuals have opted to donate their money from the Ice Bucket Challenge to other organizations. The challenge dares nominated participants to be filmed having a bucket of ice water poured on their heads and then nominating others to do the same.
The cartoon by Steve Nease shows two shops: House of Ice and Bucket World. In the shop window of the former, there's a sign saying 'Sold Out', while the latter has a sign saying, 'Out Of Stock'. The joke is that people have been buying so much ice and so many buckets for the Ice Bucket Challenge that neither shop has any supplies of either left.
Both 'Sold Out' and 'Out of Stock' mean that the product is not available, but 'Out of Stock implies that it will become available again when the next order comes in. 'Sold out', on the other hand, suggests that the product is gone, and you may not be able to get it again.