Put very simply, guerilla gardening means that you're gardening on land that doesn't belong to you, without permission from the land owner. This could mean gardening in a vacant lot, or in a street median, or in a neglected planter in a public area.
The term was first used in the early 1970s by Liz Christy, founder of the Green Guerilla Group in New York. Guerilla gardening is a radical form of gardening, undertaken by activists who often draw attention to issues such as land use rights and urban blight. Others just want to make their neighborhoods more attractive, and do it under the radar rather than going through the red tape and bureaucracy that official projects require.
Guerilla gardeners often work in the dark of night to avoid detection (this also adds to the element of surprise when a formerly overgrown public median is full of flowers or herbs the next day!) but some work during the day, so members of their community can see them at work and hopefully be inspired to do some guerilla gardening of their own.