Can anybody tell me the difference between a "1 stage" and "2 stage" Tripod?

Thanks in advance

The number of leg cross sections that extend from the main member, or leg section.

Why is '2 stage' better?

I saw a used Sachtler tripod recently, and the seller said specifically that his tripod was only 1 stage.

Thanks again :)

Less moving parts, less to wear out or break. It is also thought that the fewer sections the more rigid the legs. On better built tripods I don't find this to be a factor. Two stage tripods will setup higher, if all the sections are fully extended.

It's not intrinsically "better". Two-stage legs collapse to a slightly shorter overall length than single stage legs. They also often can be set-up lower if needed. But the trade-off is that they can weigh a bit more, can take longer to set-up and cost more (due to more materials and manufacturing complexity). Some people interpret the added cost as being added quality.