Guerilla Marketing is low cost marketing strategies that reap high rewards and they are usually unconventional ways of bringing brand exposure to your website or to your organization. For example, flash mobs or graffiti which are all unconventional. But when you are implementing marketing strategies, you do not want to expose yourself as the person that is doing the marketing.
Usually nowadays with websites, guerilla marketing can be useful in terms of getting your website viral, having good original content. For example, using the distribution channel of reddit going under desguise of being a user and having product or maybe bringing a product or an organization to the very top of the funnel where people are starting to understand what the product is before they even realize that they want the product.
That is just a quick example of what guerilla marketing really is. But again guerilla marketing is a low cost unconventional ways of bringing the maximum exposure for your website or organization.