I’m not a native but I can try to explain. Greek has two forms of expressing possession: the first is simply the genitive of the personal pronoun (in this case μου is the genitive of the first person singular pronoun εγώ). When expressing possession in this way it always follows the noun it refers to whereas the article comes before e.g. “my house” > “το σπίτι μου” and they are invariable. (Note that in English possessives are determiners and can not co-exist with articles it’s either one or the other, in Greek this is not the case). The second form is in combination with “δικός” and these behave more like adjectives and must agree in gender and number with the noun they refer to “my house” > “το δικό μου σπίτι” vs “my houses” > “τα δικά μου σπίτια”.