For example, the Danish opening:
[FEN ""] 1. e4 e5 2. d4 exd4 3. c3 dxc3 4. Bc4 cxb2 5. Bxb2
where basically, White gives up two pawns for positional play. And if you don't know the theory, you can lose by falling into traps.
So, how can you deal with this kind of gambits/traps opening, when you don't know the theory?