Some 3D art packages export their models so that the z-axis faces upward. Most of the standard scriptsA piece of code that allows you to create your own Components, trigger game events, modify Component properties over time and respond to user input in any way you like. More info
See in Glossary in Unity assume that the y-axis represents up in your 3D world. It is usually easier to fix the rotation in Unity than to modify the scripts to make things fit.
If at all possible it is recommended that you fix the model in your 3D modelling application to have the y-axis face upwards before exporting.
If this is not possible, you can fix it in Unity by adding an extra parent transform:
You have now made your mesh a Child of an empty GameObject with the correct orientation. Whenever writing scripts that make use of the y-axis as up, attach them to the ParentAn object that contains child objects in a hierarchy. When a GameObject is a Parent of another GameObject, the Child GameObject will move, rotate, and scale exactly as its Parent does. You can think of parenting as being like the relationship between your arms and your body; whenever your body moves, your arms also move along with it. More info
See in Glossary empty GameObject.