Frets are small pieces of metal or plastic, placed at carefully calculated positions on the fret board. A T-fret refers to the type of fret, which is shaped like a T and the long end is in the fret board, leaving the top as the place where the string touches it.

The top bit, called a fret board on a guitar

Action is the distance between the fret board and the strings. Make sure the strings are not too high off of the fret board, as this can make playing much more challenging.

It is the Ukulele.

pluck the strings an hold down strings on the fret board duhhh