An example description is: 1:1. That is a simple transformer with an equal number of turns in the primary and secondary. The inductance of each winding is equal to the "Base Inductance".
1:10 gives you a step-up transformer. The secondary has 10 times as many turns as the primary (so it has 100 times the base inductance).
1,2:3 gives you a transformer with two primary windings. The second primary winding has twice as many turns as the first. The secondary winding has 3 times as many. The only difference between primary and secondary windings is how they're displayed on the screen.
1:.5+.5 gives you a tapped transformer.
1:1+2 gives you a tapped transformer where the first part of the secondary has as many turns as the primary, and the second part has twice as many.
1,-1:1 gives you a transformer with two primary windings, where the bottom winding has its polarity swapped.
So, the description is a list of windings separated by commas, with : to separate primary and secondary, and + to separate parts of a tapped coil.