Simply put, it means feeding your baby whenever she's hungry. If she cries, don't look at the clock to try to gauge whether she "should" be hungry. If she is, then it's fine to feed her. Babies don't eat out of boredom or habit; they eat when they need to.
Right. In previous generations, babies were fed according to the clock, often every three or four hours. If they cried in between they were soothed, but not with food. The theory was that babies and mothers do better with a predictable routine. While that's certainly still true, modern child development theory is that babies should be given food whenever they're hungry, so they grow up secure in the knowledge that their most basic needs will be attended to. Bath time, bedtime, and other daily rituals work well if you set a routine, but a baby who wants food should be given it.
Before crying for food, a baby usually grows squirmy, roots for a breast, smacks her lips, or even makes sucking motions. These are all indications that she'd like to be fed. Once she starts crying, she's almost certainly hungry, though of course if you offer her the breast and she's not interested, it could be that something else is bothering her, from gas to a wet diaper.
To outsiders, the notion that you may need to feed your baby every hour and a half can seem ludicrous. You may have to educate them about the fact that breast milk is digested quickly, and some babies need to drink often, especially newborns with their tiny stomachs. You might respond by saying something like, "She needs to eat often now, but it won't be like this forever," or "The doctor suggests I feed her whenever she's hungry, which is pretty often. Once she grows a little bigger she'll be able to go longer between feedings."
Sources: American Academy of Pediatrics; La Leche League
All content here, including advice from doctors and other health professionals, should be considered as opinion only. Always seek the direct advice of your own doctor in connection with any questions or issues you may have regarding your own health or the health of others.