。
I don't understand what you're saying.
Literal
I [topic-は] you [possessive-の] saying [nominalizer-こと] [subject-が] do-not-understand.
言っていること ('what you are saying') — 言う in ~ている (ongoing) + こと (nominalizer). がわかりません = polite negative of わかる ('to understand'). A very common expression for expressing confusion.