An Isosurface 
is a surface along which a solution variable has a constant value. When an isosurface is created on a plane, it will be in the form of a line and is generally called a contour. You can use the isosurface tool to visualize the boundaries between specific regions of the solution domain.
Click the isosurface icon
in the visualization toolbar. This will open up the tool in the properties panel.Select the desired flow quantity from the Isosurface Field dropdown.
Create isovalues by entering values into the Isovalue box or using the Value Generation fields. Minimum and maximum values of the chosen flow variable are shown below the box to help you select appropriate numbers.
Click the check button to apply the settings.
Navigate to the Visualizations section of the control panel and select the Isosurface that you just created.
Use the dropdown menus in the Display section to adjust the final appearance of the isosurface.