Ritual fasting for Catholics is one modest meal per day; you are permitted two smaller meals that together don’t equal that one modest meal. (I’m inserting the word modest because some people go overboard and miss the point.) Individuals will take things a little further as they have discerned appropriate for themselves, often with guidance from a spiritual director. Others are not going to be able to keep to this simply due to health, and that’s perfectly ok.
Edit to add: you’ll find disagreements over liquid intake and what “counts” and doesn’t toward breaking the fast. For some, water only is permitted; for others, any essentially non-caloric liquid is ok. The Church doesn’t strictly define these things for us as the intent of the fast is more important than the actual details.
