If you're an open and straightforward person, you're upfront. And if your cousin asks what you think of his weird new haircut, you'll be upfront with him and tell the truth.
If you prefer being flattered, you might not appreciate your brother's upfront pronouncement that your new pants are way too small or you've got spinach in your teeth. But most of us like knowing that our friends are being upfront and honest with us. Another way to use this adjective is to mean "in advance" when you're talking about a payment: "She wants the cash upfront, before she hands over the cupcakes."