Hi there,
From reading answers to this question on similar threads, this is what I think it means:
Your tamagotchi has visible indicators (eg. hunger meter, happiness meter, discipline meter) and hidden indicators (eg. body needs, heart needs). In order to get certain characters, you need to take care of or neglect certain indicators depending on what character you want.
The visible indicators are pretty self-explanatory: feed it to fill hunger meter; play with it/give it a snack to fill happy meter; discipline/praise it appropriately to fill discipline meter.
The hidden indicators are slightly more tricky and are affected by how well you take care of the visible indicators and other tamagotchi needs.
To fullfill its body needs, feed your tamagotchi as soon as a heart in the meter empties, give your tamagotchi medicine at the first sign of illness, and time out your tamagotchi when appropriate. For best results, do each of these things within 15 minutes of the need arising.
To fullfill its heart/spiritual needs, fill a heart on the happy meter as soon as it empties (within 15minutes), switch off the lights when tamagotchi goes to sleep (within one hour), and praise it when appropriate (within 15minutes).
Also, achieving a "Great" in the jump game helps fullfill its body needs and achieving a "Great" in the dance game helps fullfill its heart/spiritual needs.
Hope this helped. There are different explanations of this throughout tamatalk so if you're confused by my explanation then check out some others. Admin posted an excellent and comprehensive explanation of the hidden indicators in the Tips and Tricks section titled "Hidden Parameters". I think its on the 2nd page.
Good luck!