It sounds like your fellow community members covered most of it, but I’ll clarify on a few! :)
1.) In simple terms, the altimeter calibrates the aircrafts altitude. As the barometric pressure changes, so does the altimeter. It is important the pilot has entered the correct values on their altimeter so they’re showing the correct altitude. Imagine how important this is when there is low visibility and the pilot believes he is still at 500ft, but in all actuality he is about to hit the ground.
2.) The “code” correlates to the current ATIS code. Prior to contacting ground or any receiving facility, the pilot will check the ATIS to find all pertinent and current information pertaining to that airfield. By reciting the correct ATIS code, the controller knows the aircraft has the most current ATIS that is being broadcasting.
3.) IFR and VFR simply describe the weather conditions and type of flight.
IFR weather: Ceiling below 1000ft/Visibility less than 3SM
VFR weather: Ceiling at or above 1000ft/Visibility at or above 3SM
Along with weather also comes how aircraft will classify themselves during the flight. If an aircraft is flying VFR, they’re proceeding to their destination visually, providing their own separation by the basic VFR means of “see and avoid”. If an aircraft is flying IFR, they’re receiving all instruction while airborne from ATC, to include vectors, altitudes, etc. The weather does not need to be IFR to be flying IFR.
4.) Clearance Delivery is used by pilots to receive important information from ATC prior to contacting ground. Before any flight the pilots must file a flight plan with ATC. ATC then scrutinizes the pilot requests and approves or amends the request based on traffic volumes, weather, destination conditions, etc. They then read the clearance to the pilot with information like route of flight, squawk, altitude to expect, departure frequency, and many others.
5.) Ramp frequencies are often times used to request a pushback at bigger airports where ground does not manage the individual ramps. A ramp controller will approve a pushback and taxi up to but not beyond the ramp hold line. From there the aircraft will request taxi instructions to a departure runway. Each airport is different. Where I work the ramp frequency is used to communicate with airfield operations vehicle and many other airfield agencies.
Feel free to continue the thread or PM if you ever have questions. Don’t forget google.com and youtube.com are your best friend for some of the basic aviation procedures. I’ve learned a lot there even outside of my formal training!