In short
Berberine is a bitter-tasting alkaloid compound found in several plants (including barberry and goldenseal), long used in traditional Chinese and Ayurvedic medicine, that activates AMPK and has been studied for effects on blood glucose and lipid metabolism.
Berberine's proposed mechanism overlaps with metformin's in some respects — both are thought to activate AMPK, partly via mild inhibition of mitochondrial complex I — which has led to berberine being popularly nicknamed "nature's metformin" in wellness marketing, though the two are chemically unrelated and this label overstates how comparable their evidence bases actually are.
A number of small-to-moderate human randomized trials, mostly conducted in China, have found berberine (typically 500 mg two to three times daily) improves fasting glucose, HbA1c, and lipid markers in people with type 2 diabetes or metabolic syndrome, with effect sizes in some trials approaching those seen with standard oral diabetes medications. A 2012 meta-analysis (Dong et al., Evidence-Based Complementary and Alternative Medicine) pooling several of these trials supported a meaningful glucose-lowering effect, though study quality and size in this literature are generally more limited than the large, multinational trials underpinning approved diabetes drugs.
Berberine's most common side effects are gastrointestinal (diarrhea, cramping), and it has clinically relevant drug interactions — it inhibits cytochrome P450 enzymes (notably CYP3A4 and CYP2D6) and can raise blood levels of drugs metabolized by these pathways, which matters particularly for anyone also taking other blood-sugar-lowering medication, given the added risk of additive glucose-lowering effects. It is not FDA-approved as a diabetes drug and should be treated as a supplement with a genuinely more substantial human evidence base than most in this category, but not as a validated substitute for metformin or other prescribed diabetes therapy.