Translation:Mom likes to drink juice in the afternoon.
Good question! Historically both אחר and אחרי were used and meant the same ("after"). In modern Hebrew generally only אחרי survived. אחר is used, as far as I can recall, only in two set phrases: אחר כך ("afterwards"), and אחר הצהריים which is used alternatively with אחרי הצהריים. Maybe אחר הצהריים is a tiny bit more formal or "old-ish".
Hebrew splits the day differently from English. "Morning" is until 11 AM or so, than צהריים is from 11 AM to 2 PM or so, then it's אחרי הצהריים until 6 PM or so, then ערב. The boundaries may change considerably depending on social habits, but these are the parts. You can imagine the challenge in translating צהריים to English in texts, and for native Hebrew speakers to express their צהריים thought in English...