What does it mean by eccentricity in column ? can someone explain it to me ? why does it happen?

It refers to the forces or loads on the top and the bottom of the column. They should be in the same vertical axis. In the case of an eccentrically loaded column the two forces are not in the same vertical axis. One is offset from the other. This encourages the column to buckle and therefore limits the load the column can hold up.