I presume you're asking about the difference between describing something as 'so nice' as opposed to 'very nice'?
In spoken language and informal writing, the 'so' option is stronger than 'very', as well as being more enthusiastic and more emotional. 'Very' can sound a bit too cool and even slightly negative in spoken language.
The 'so' option isn't appropriate for written language, other than for informal correspondence. We'd use 'very' in most written contexts.