Dear Lubenka,
An aneurysm is a "sac or dilation" of a structure of the body. The intratrial septum is the wall between the two upper chambers of the heart. Aneurysms in this area appear as a "floppy" intra-atrial wall. This is sometimes associated with a hole or shunt between the two chambers but this was not detected in your case. In general this is not something which needs treatment.