Scope and limitations, as part of a thesis, work to outline the boundaries of the thesis in question.
Often, the scope and limitation of a thesis will be presented in a small chapter within the thesis, usually directly after the table of contents, and it's purpose is to define the parameters in which a thesis will be operating. Scope primarily refers to how much of a certain topic the thesis will be focusing on, whilst the limitations serves to highlight areas where the thesis might not develop into. No matter how much research, time and writing is involved in your thesis, there will always be related areas that you will choose not to stray into or will not be able to expand into for various reasons, so including this in your limitations shows that you are fully aware of this, and helps counteract any arguments against the strength of your thesis.
The best advice when writing your scope and limitations chapter is to practice a certain amount of humility; be honest, practical and objective- in all probability your paper will be read in detail by a panel of experts in your particular field, and recognizing the boundaries of your work can often play a key role in showing your understanding of the field in question.