I began to study organic reactions and found the term "attack" everywhere, what this "attack" precisely mean?
You might have heard the term "attack" in statements like "the nucleophile attacks the electrophile", and terms like frontside and backside attack, and what this basically means is that the nucleophile approaches the electrophile, and begins to share its electrons with the electrophile to form a bond between them (and possibly break a different bond in the molecule too). Frontside and backside attack mean the nucleophile approaches the front or the back of the molecule and forms a bond.
If you've learnt about arrow pushing, you'll know that an arrow has its tail starting at the nucleophile (the electron rich species) and it's head points to the electrophile (the electron poor species) - you can also think of "attack" as the nucleophile "attacking" the electrophile with electrons to react/form a bond, in the direction the arrow is pointing. I suppose we call it attack because it makes it sound more action-packed :-)