what is difference between sierra and high sierra

Sierra is macOS version 10.12. High Sierra is its successor, macOS version 10.13. Sierra was introduced in September 2016. High Sierra was introduced in September 2017. High Sierra is said to be a refinement of its predecessor, focused on performance improvements and technical updates rather than new user features.

The hardware that is capable of running Sierra can also run High Sierra. However, Sierra has now dropped from Apple's customary three-year, sliding support window, so no further security or browser updates will be forthcoming for it. High Sierra will continue to be supported until Fall 2020 when macOS.next is released.

As vendors' re-evaluate their product support criteria, you may find that newer product updates require a newer version of macOS than Sierra. Microsoft's own support criteria is the same sliding-window that Apple uses in the first paragraph. Right now, one is required to have High Sierra before any Office 365, or Office 2019 for Mac installation is permitted. That will change to Mojave (10.14) in Fall 2020.

The new Apple file system APFS was introduced on High Sierra. Other than that there are no truly significant differences between Sierra and High Sierra. I upgraded to High Sierra on one machine only because I wanted to lose my old MS Office and move on the Office 2019. I didn't go all the way to Catalina because there are some 32-bit programs I still want to have access to. I could just as well upgraded to Mojave.

Sierra is not going to run on an APFS drive like High Sierral. If you are using Microsoft Office 2011, then Sierra is ok but definitely not High Sierra no matter who says what about it.