The definition of a substance that is "multi-phase" is that there is more than one distinct compound in it and the compounds form distinct regions in the substance with different properties.
Examples make this clearer. Salt dissolved in water forms a uniform mixture: a salt solution. There is more than one compound there but they are perfectly mixed.
Fatty organic compounds are not soluble in water but can form small dispersed fatty particles. We call the result milk (I'm simplifying a little as there are other things in there as well). The result has more than one phase: a microscope would reveal very small, dispersed particles of fat floating in water. Milk looks like a single uniform substance, but isn't when you look more closely.
Fibreglass is, essentially, a bunch of small fibres made of glass embedded in a matrix of some polymer. There are two distinct phases with different compositions in the substance.
The characteristic of multi-phase compounds is that different regions of the substance contain different compounds (these regions can be very small as in some metal alloys). In your case the reaction seems to produce two distinct compounds which may be finely mixed together. The XRD pattern shows two different crystal structures. This may result because this specific reaction does not create the compound you wanted under these conditions. Which could be because the compound you want is not stable or could be because the reaction conditions allow more than one product to form. Either way the reaction is giving a mixture not a single compound.