A verb is irregular if its past tense and past participle do not follow the regular pattern of adding -ed (or -d) to the base form.

Regular verbs:

arrive – past tense arrived, past participle arrived
cook – past tense cooked, past participle cooked

Irregular verbs:

eat – past tense ate, past participle eaten
lose – past tense lost, past participle lost

Most native English speakers have a good grasp of which verbs they can’t stick -ed on the end of. To native ears, forms such as I catched or I have readed sound childish or unnatural. It isn’t always as obvious for non-native speakers.

Native speakers do sometimes find it difficult to pick the correct form for past tense and past participle. For example, is rang or rung the past participle of ring?

Simple present: I ring
Simple past tense: I rang
Past participle: I have/had rung

There isn’t really a rule or tip I can give to help here, except maybe to list all the forms. And so that’s what I have done. You can download a comprehensive (I think) list of irregular verb forms by visiting my Resources page or clicking this link: Irregular verbs.