A fricative is a consonant sound that is created by constricting the vocal tract, causing friction as the air passes through it. The nine English fricative sounds (the v sound, f sound, voiced th sound, unvoiced th sound, z sound, s sound, zh sound, sh sound, and h sound) often do not correlate exactly with any particular sound in an English as a Second Language/English as a Foreign Language student's native language. This causes substitutions to occur, and those substitutions often have significant differences from the intended English sound.
There are three major points that beginner ESL/ELL students should understand about fricative sounds:
To produce fricatives, air travels smoothly through a small, constricted opening in the vocal tract
Fricatives are capable of being formed continuously, with no complete blockage of the vocal tract
Except for the h sound, fricatives occur in voiced/unvoiced pairs