I worked at tesco for 13yrs i recently got the sack for gross misconduct and now i need to get a reference for a new job do Tesco have to explain the reason why i left my role or not in my reference
If you were fired, then Tesco will probably mention it.
What exactly they say may depend on what the new employer asks.
If the new employer just asks for a general reference, then what exactly Tesco say and how much detail they go into regarding your sacking will be up to them.
Some employers ask specific questions in references, regarding stuff like your suitability for the job etc etc. If so, they usually ask a question in there about any warnings or other disciplinary action, in which case tesco will have to mention it.
Or Tesco could just choose to not give you a reference at all. That's up to them.