A pillar article is a tutorial style article aimed to teach your audience something. Generally they are longer than 500 words and have lots of very practical tips or advice.
This article you are currently reading could be considered a pillar article since it is very practical and a good “how-to” lesson. This style of article has long term appeal, stays current (it isn’t news or time dependent) and offers real value and insight. The more pillars you have on your blog the better.