I need to understand the meaning of this word "2 1/2 turn ferrite bead" ?? What exactly this 1/2 means ? Does this means that the wire is turned twice inside the bead?
In this terminology you have encountered,
One turn of a coil is the wire making a full loop and ending up where it started.
Half a turn is the wire passing through the core without looping.
So for a 2 1/2 turn coil, you put the wire through the the core/bead three times. A half turn by itself is a ferrite bead on a straight wire.
In general, an “and a half” coil has the two ends of the wire exiting from opposite sides. A “whole number of turns” coil has both ends on the same side.
Example of 2 1/2 turn coil (photos from this question); observe that the ends of the wire are on opposite sides of the board:
Example of 1/2 turn coil, a bead on a wire (taken by Wikimedia Commons user Omegatron):
I'd like to add some photos of "whole turn" coils but couldn't find any suitably licensed and I don't recall which piece of my own equipment to disassemble to find one :)