"i" is for "initial"
So an i12 is an attack that has 12 start-up frames. oN the 13th frame, it will transition into "active" frames, meaning if those frames hit the opponent, they receive damage. After the active frames end, it goes into "recovery" frames, or the frames it takes after the active frames while the move is locked in animation before another action can take place.
So an i12 is an attack that has 12 start-up frames. oN the 13th frame, it will transition into "active" frames, meaning if those frames hit the opponent, they receive damage. After the active frames end, it goes into "recovery" frames, or the frames it takes after the active frames while the move is locked in animation before another action can take place.