Good question.

Wikipedia puts it rather well, as:

A solidarity economy is based on efforts that seek to increase the quality of life of a region or community through local business and not-for-profit endeavors. It mainly consists of activities organized to address and transform exploitation under capitalist economics and the large-corporation, large-shareholder-dominated economy, and can include diverse activities.

The article goes on to talk about strategies for "humanizing" the economy – doing business and economics as if people mattered.

This is what we are about. We're inspired by programmes like the Evergreen Cooperatives in Cleveland, Ohio, and like the work happening much nearer to us in Preston in Lancashire where they have relocalised tens of millions of pounds of public sector expenditure, massively increasing the positive local impacts of all that spending. You may have seen references to the so-called "Preston Model". This is about 'community wealth-building', and understanding that contracting out to the lowest bidder can sometimes be a very poor strategic choice in terms of local economic impact when that winning bidder is a distant corporation with no interest in your local economy other than extracting value from it.