"Fat burner" is a broad category name rather than a single ingredient - most products in this space combine a handful of stimulant and metabolic ingredients (commonly caffeine, green tea extract, and similar compounds) intended to modestly increase energy expenditure or support focus during a calorie deficit.
It's worth setting expectations correctly: no supplement replaces the fundamentals of fat loss, which come down to a sustained calorie deficit, adequate protein, and consistent training. A fat burner is, at best, a small edge on top of those fundamentals - more useful for energy and appetite management during a cut than as the main driver of results.
Because these products are often caffeine-based, it's worth checking your total daily caffeine intake (including coffee, pre-workout and energy drinks) before adding one in, and avoiding them late in the day if you're sensitive to stimulants or they affect your sleep.
If you're building a weight management routine, see our guide to reducing body fat for the fundamentals, and our Whey Protein Isolate for supporting protein intake through a deficit.
As with any stimulant-containing supplement, check with your healthcare professional before use, particularly if you have any heart conditions or are sensitive to caffeine.