An irregular verb is one which does not follow standard rules of conjugation in the different verb tenses. The verb "to want" is a regular English verb: present tense=want, past tense=wanted. The English verb "to be" is an irregular English verb, as its past tense is not 'beed, but 'was'. In Spanish, regular verbs follow the same rules of conjugation in all the fourteen (!) Spanish verb tenses.
There are about twenty irregular Spanish verbs. In Spanish as in English some of the most important and most frequently used verbs are irregular. Hence it is necessary to learn how they are conjugated, primarily in the most common verb tenses- the simple present ( English: 'I go' )and the simple past (preterit) (English: 'I went'). Once these irregular Spanish verbs have been memorized, the rest of your Spanish study will be a relative cakewalk.