If there's anything you don't understand, ask me anytime.
Literal
Not-understand thing [subject-が] existed-[if-たら], anytime me [target-に] ask.
~ことがあったら combines nominalization (こと), existence (ある), and the ~たら conditional — 'if it should happen that there's something you don't understand.' いつでも (anytime) reinforces open availability. A warm, encouraging sentence from a teacher or senior.