que es "verbo imperfecto"
"Un verbo imperfecto" is a verb in the imperfect tense, as opposed to the preterite. The imperfect tense is one of the past tenses in Spanish and is (very roughly) used to refer to things that used to happen or were happening. Here's a video and some practice materials on when to use the imperfect and preterite: http://www.spanishdict.com/learn/show/27.
verbo imperfecto-verb in the imperfect tense
Here's the reference article on the imperfect.