Translation:We have followed Pedro all day.
This is the present perfect tense. You are saying we have followed, not we follow. Present perfect is formed by conjugating the helping verb haber in the present tense (here, for we, it's hemos) followed by the past participle of the main verb (seguir --> seguido). There are some exceptions, but past participles generally are formed by removing the verb ending and adding -ado (ar verbs) or -ido (er/ir verbs).
Most likely it's because of the negative:
"No ha llovido en todo el día." - There was not a single point within the day where it has rained. It's about a specific point in time.
"Ha llovido todo el día." - It has rained all day, all the time. It's not a specific point within the day, but the day itself, basically.