Documentation on the website which also happens to be the first search result for the term SetTextureOffset or mainTextureOffset seems to sum up the issue nicely.

Material.SetTextureOffset

function SetTextureOffset (propertyName : String, offset : Vector2) : void

Description
Sets the placement offset of texture propertyName.

Common texture names used by Unity's builtin shaders: 
"_MainTex" is the main diffuse texture. 
This can also be accessed via ***mainTextureOffset*** property. 
"_BumpMap" is the normal map. 
"_Cube" is the reflection cubemap.

Material.mainTextureOffset

var mainTextureOffset: Vector2;
Description

The texture offset of the main texture.

The same as using GetTextureOffset or SetTextureOffset with "_MainTex" name

A material has a main texture which is the one you see, additionally is can have other textures such as a bump or cube map which effect how the objects appearance is calculated. mainTextureOffset only effects the materials main texture. SetTextureOffset lets you specify the name of the texture to offset, so SetTextureOffset("_MainTex" .05) would be eqivelent to mainTextureOffset = .05;

Documentation on the website which also happens to be the first search result for the term SetTextureOffset or mainTextureOffset seems to sum up the issue nicely.

Material.SetTextureOffset

function SetTextureOffset (propertyName : String, offset : Vector2) : void

Description

Sets the placement offset of texture propertyName.

Common texture names used by Unity's builtin shaders:

• "_MainTex" is the main diffuse texture. This can also be accessed via mainTextureOffset property.
• "_BumpMap" is the normal map.
• "_Cube" is the reflection cubemap.

Material.mainTextureOffset

var mainTextureOffset: Vector2;

Description

The texture offset of the main texture.

The same as using GetTextureOffset or SetTextureOffset with "_MainTex" name

A material has a main texture, which is the one you see. Additionally, it can have other textures, such as a bump or cube map which affect how the object's appearance is calculated. mainTextureOffset only affects the material's main texture. SetTextureOffset lets you specify the name of the texture to offset, so SetTextureOffset("_MainTex", .05) would be eqivalent to mainTextureOffset = .05;