I read a blog post about itp. But I'm still not clear - what is it?

Low platelets. Itp stands for immune (or idiopathic) thrombocytopenic purpura, a fancy term for a low platelet count with bruising. This is a disease of the immune system, where platelets are targeted for destruction and removed by the spleen. People have varying degrees of low platelets. Treatment will depend on how low the platelets are, and how severe bruising is.
ITP. It is an autoimmune disorder where your body's own antibodies act the platelets. If it becomes overwhelming and the platelet numbers drop too low, bleeding can occur. If any concerns, you should see a hematologist.