I'm not sure if this is the answer that you're looking for, but 'in' means (more or less) 'in' in English, while the forms 'nel' and 'nella' mean 'in the'.
'Nel' is a compound form made up of 'in' and 'il', while 'nella' is a compound made up of 'in' and 'la'.
So, for example, if you want to say 'in the park', rather than 'in il parco', you would say NEL parco.
Likewise, if you wanted to say 'in the kitchen', rather than 'in la cucina', you would say NELLA cucina.
The forms 'nello' and 'negli' also exist as the compound forms of 'in' with 'lo' and 'gli'