Your husband has just come home. He is looking around the kitchen for something to eat. There's a pot of soup on the stove which you made earlier today. He asks about it. You say:
It's still good. Just heat it up a bit.
When you're talking about food, the word "good" can have more than one meaning. One meaning of "good" is "delicious":
The other meaning of "good" is "not rotten" or "not stale". This is the way that "good" is used in the example at top.
To "heat ___ up" means to make something warm enough to use. You "heat up" cold food or a cold room:
It's freezing in here! Do you want to turn the heater on and heat it up?
The word "heat" by itself also means to make something warm, but it sounds a little too technical to use when you're speaking conversationally about food. However, it's perfect to use for a written recipe:
Heat the oil in a large skillet over medium heat; add the onions and cook until brown.
"A bit" means "a small amount" or "a short time":
In the example at top, it's not really clear whether "a bit" means "a small amount (of heating)" or "for a short time". But the basic meaning is the same either way.
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