A non-GAAP measure is a financial figure that is not defined by generally accepted accounting principles (GAAP). Companies define their own: adjusted earnings, adjusted EBITDA, free cash flow, organic growth. They usually take items out, most often stock-based compensation, amortization of acquired intangibles, restructuring charges and one-off gains or losses.
The rules
When a company uses a non-GAAP measure in public, it must also present the most directly comparable GAAP measure, and reconcile the two. In filings and earnings releases, the GAAP figure must be given equal or greater prominence.
Reading them
The reconciliation table is the useful part. It shows exactly what the company took out. An item called "one-off" that appears every year is a cost of doing business. Compare the adjusted figure with the GAAP one over several years, not just this quarter.
On Listed
Listed uses GAAP (or IFRS) figures from XBRL data only. It does not show companies' non-GAAP measures. Its own free cash flow uses one fixed formula for every company, which can differ from a company's own definition.