Cutoff and Resonance Filter
Cutoff is used to block certain frequencies from a sound. In a standard LP (Low Pass) cut/reso filter, cutoff determines the range of high frequencies to be cut. However, in modern software and hardware synthesizers (and in FL Studio itself) you will notice that cut/reso filter comes in many "flavors", such as HP (High Pass), which cuts the low frequencies of the sound instead.
Resonance effect originates from an artifact in the original cutoff filter used in old hardware synthesizers. It is narrow band of frequencies, near the cutoff level, where the sound is amplified. Today this artifact can be easily avoided, but it is still available though, because it can be used as a special effect. Changes in the cutoff level together with high resonance produces interesting phaser-like effect, which is one of the reasons for the popularity of the TB-303 synthesizer.