I don't understand the meaning of "you're enough to use me for good". Could you explain?

Please let me know how to use "in a jiffy".

"In a jiffy" is an expression that means very quickly, fast, or soon. You can use this phrase whenever you think you can do something in a short amount of time. Here are a few examples. Ex1: Don't worry, I can get that paper done in a jiffy! Ex2: I'll have this car fixed in a jiffy. Ex3: These clothes will be washed in a jiffy.

I don't understand the meaning of "you're enough to use me for good". Could you explain?

Although it's not very clear in the song, my guess is that "you're enough to use me for good" means that his lover is "special enough" that he has given his lover the power to use him forever. Note that "for good" means "forever," "permanently," or "definitively," so the meaning of "you're enough to use me for good" is "you're enough to use me forever." To be "enough" means to be good enough, special enough, for something. Thus, I think the singer is trying to express that his lover is special enough to him that he has given his lover a lot of power over him and his actions. Ex: Once we get married, you're stuck with me for good. Ex: Don't worry about what others say. You're enough.

What does "point" mean here?

"Point" here is short for "point in time". Ex: At some point, I should do my chores.

What's "keep fit" mean? And isn't "fit" similar to "suitable?"

You're right. "Fit" can mean suitable! However, here it means healthy or in good shape. Implying that the person exercised. So, "keep fit" means to continue to stay or work on being "fit." Ex: I'm not sure how I'm going to keep fit while in quarantine. Ex: She kept fit by joining a local football club, going on jogs, and eating well.

Is "switch the lights off" different from "turn the lights off"?

No. These two phrases are fairly interchangeable and have the same meaning. Here are some example sentences of both being used. Ex: Can you turn the lights off before you leave? Ex: Can you switch the lights off before you leave? Ex: Don't forget to turn off the lights. Ex: Don't forget to switch off the lights.