An instance can be flagged as solid through the object
properties, or by changing the value of this variable. If
solid is true, a special collision event is generated
whereby the instance is returned automatically to the position it
was at in the step previous to the collision. If it is set to
false, all collisions must be dealt with through the collision
solid = !solid;
The above code checks the "solid" flag of all instances that have the same object_index as that which is running the code and if "solid" is false it is changed to true.