The intension of a word or phrase may be regarded as a description, or set of properties (if one can indulge talk of properties at all), which applies to each member of the word's extension, and which distinguishes those things from everything not in the extension.
This is a rather complicated definition, but it is not a difficult concept. What, for example, would be the intension of the phrase 'Wikipedia readers'? Simply the property of having read any part of Wikipedia at any time. Anything that has that property is part of the extension of the phrase 'Wikipedia reader'. For example, Jimbo Wales has been reading Wikipedia; so the intention of the phrase 'Wikipedia reader' applies to Jimbo Wales; and so Jimbo Wales is part of the extension of 'Wikipedia reader'. Moreover, anything that has not read any part of Wikipedia at any time is for that reason not part of the extension of the phrase 'Wikipedia reader'.
The above account is provided just in order to offer some basic understanding of what 'intension' amounts to; in fact, what intensions are is a matter of some of the most difficult and extensive debate, going to the heart of questions about meaning and content.