Like the other poster said, there's a "probation" for retained trainees in Big4 firms - if you leave during this "probation" period, you will be asked to pay the firm back for your Part B fees. When I trained at a big4 firm a couple of years back, this period was 6 months. This is the lower end, and other firms impose longer periods.

As for the "probation" viz employment, generally the period is around 6 months - this usually applies for lateral hires, rather than retained trainees. In all honesty, since they already know your working style and your fit, and because they're saying that they are fine with your work performance, this "probation" is probably just the firm trying to protect itself and have the option to let you go should it not be able to afford your salary.