Verbs have four principal parts: (1) the infinitive, (2) the past, (3) the past participle, and (4) the present participle. A regular verb forms its past tense and past participle the same way, usually by adding -ed. Take the regular verb walk. The past tense is walked, as in He walked to the store. The past participle is also walked, as in He has walked to the store (the perfect tenses are formed by using have as an auxiliary and combining it with the past participle).
The verb keep is also regular. It forms its past tense (kept) and its past participle (kept) the same way.
But an irregular verb has one word for its past tense and another word for its past participle. They don't just add -ed. Typically, they change an internal vowel (I drink, I drank, I have drunk), or they add ‑en to form the past participle (I choose, I chose, I have chosen).