can anybody tell me what is tri tolerance in ICEM HEXA?

Tri Tolerance is a measure of how well the geometry (tetin file) is tessellated when read into the Mesh Editor. Typically the default value is good enough. However, if you're using very fine near-wall spacing, sometimes you need to reduce this. Change the value, then save and re-load the tetin file to make the change in active memory. This takes more memory to represent the geometry, but it will be truer to the actual shape.