What is meant by limit state?

A limit state is a condition of a structure beyond which it no longer fulfills the relevant design criteria. The condition may refer to a degree of loading or other actions on the structure, while the criteria refer to structural integrity, fitness for use, durability or other design requirements.

What is difference between SLS and ULS?

SLS = Serviceability limit state. It considers how the structure is actually expected to behave, generally by using partial factors of 1. ULS = Ultimate limit state. It considers complete structural failure, utilising partial factors to increase loads and decrease strength.