I have a linestring which I need to buffer but the problem is that I am not able to understand the unit metric in which the buffer function accepts the linestring.
From Shapely's project page:
Shapely is not concerned with data formats or coordinate systems, but can be readily integrated with packages that are.
so @Jon's answer is correct. The unit depends on the unit of your input data. You can use geopandas, pyproj or GDAL to convert your lineString to another crs.