Basically it's a player who decides to let his contract with his club run out, so he can leave them without another team paying a transfer fee.
This leaves the way open for the team who want to sign him therefore having more money to pay his now vastly-inflated wages, only for him to do the same to them when his contract runs out again.
It has also helped in the total loss of loyalty towards a team that was once shown (how many players spend their whole career at just one club nowadays), and IMHO a general decline in football as a whole in the last however many years since it was introduced.
Rant over