Hi Sofia,
I presume you have applied and they have invited you to do the online test?
They give you a sequence of numbers and letters, and give you 5 x answers, one of the answers is the same as the sequence initially given and you have to match them up.
Think there are 5 sets of 5 questions and the time limit goes down on each one. They give you some test ones before hand though so I wouldn'y worry.
Good luck and let me know how you get on.