“It's still good. Just heat it up a bit.”

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 respond like this.

It's still good. Just heat it up a bit.

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(something) is still good

When you're talking about food, the word "good" can have more than one meaning. One meaning of "good" is "delicious":

Wow, this is actually pretty good.

The other meaning of "good" is "not rotten" or "not stale". This is the way that "good" is used in the example at top.

heat (something) up

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.

(do something) a bit

"A bit" means "a small amount" or "a short time":

Well listen, I'm going to go mingle a bit.

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.