Module Position; this is a grid of possible positions into which you can assign modules.
The grid varies from one template to another.
When viewed on screens of differing sizes, the module positions re-arrange themselves to fit the screen. This feature is called 'responsiveness'.

Hi Martin,

Thanks for that

I'd go with something a bit more

Module positions are placeholders in a template

Modules positions are dictated by the template layout as laid out by the template developer

Modules positions can be loaded in to articles using the {loadposition} syntax

Modules (using their positions) can be displayed at various widths depending on the template settings

Modules positions can be hidden using css classes e.g. hidden on chosen device widths

Just to explain

You can target module positions width using the different screen widths in the T3 framework
www.joomlabamboo.com/blog/how-to-joomla/how-to-resize-position-widths-in-t3-templates

Its listed under responsive layouts section
It also explains how to hide positions

For finding out about both loadposition and loadmodule take a look here

www.ostraining.com/blog/joomla/loadposition-loadmodule/