Answer:
In a solution you have a substance dissolved by the solvens. Thus you have molecular interactions between different molecules in order to create a solution.
Example: Water molecules break the ionic grid of a salt and form and create a hydrate shell around the separated ions.
A mechanical mixture is when you simply mix different materials without any interaction.
Example: You take two different Salts and mix them. While you may not be able do see wich grain is from wich salt, they are just mixed, not dissolved or something.