In this usage immediately in front of a verb, it has no translation in Modern Welsh. Along with the alternative fe, it can be used to indicate that the verb is being used in an affirmative statement, not a question or a negative. fe/mi causes a soft mutation of the verb.
The use of fe/mi in this way varies across Wales and between different people and situations, too. Unless you are speaking to people who do use them it is probably easiest not to use them, but just to be aware of them.