"I want to go to the office on Wednesday."
Translation:Dw i eisiau mynd i'r swyddfa dydd Mercher.
Here, i is a preposition 'to' - mynd i'r swyddfa = going to the office. It is also a preposition (= to, for, in order to, ...) in patterns such as rhaid i chi fynd (= a need for you to go), dyn ni'n mynd i siopa (= we are going shopping), dyma arian i ti (= here is some money for you).
With first person singular verbs, for example, it means 'I' - gwisga i grys (= I will wear a shirt), roeddwn i'n dlawd (= I was/I used to be poor), bues i yno (= I was there).
ibischris told you what it means - the answer to "what's it short for?" is that i'r is short for i yr, i.e. preposition i + definite article yr.
(Though note that the article is y before nouns starting with a consonant -- but i'r is used before any noun, whether starting with a consonant or a vowel -- as regularly happens when the article is tacked onto a previous word that ends with a vowel and takes on the form 'r.)