less is a terminal pager program used to view (but not change) the contents of a text file one screen at a time. Questions regarding the use of less in Ubuntu should go by this tag

less emerged from a necessity of a pager for large files. vi couldn't open files over a certain size while more was impractical for navigate the files. This prompted Mark Nudelman to write less - opposite of more. Usage of this tool should use this tag.

Less was first written in 1983 by Mark Nudelman, and is the current maintainer of the project. The first public release was done in May, 1985 in the net.sources UseNet.

Why is it called less?

It is better that you read the author's FAQ, but in summary, it is about a joke that they needed "backward more" and someone he doesn't remember said that they need "less", hence the term was coined for the tool.