What does the な in the center means.Is it a particle.
The word 便利 is a so-called な-adjective. When な-adjectives directly modify a noun, な has to be added between it and the noun it's modifying (hence the name).
So, "便利な工具" means "a handy/useful tool".
As for your second question: You can call it a particle in the sense that it's a small word which serves a grammatical function, but I don't know if a linguist or grammarian would actually classify it as such.
便利｛べんり｝ is a な-adjective, which is used to modify a noun, in this case, 工具｛こうぐ｝. When modifying a noun, な must be placed after な-adjectives (便利｛べんり｝) and before the noun it modifies(工具｛こうぐ｝).
The phrase 便利な工具 would translate then, to "convenient/useful tool(s)"
I am not sure if most would consider the な a particle, or simply a suffix used with な-adjectives.
Another name for this type of words is "adjectival nouns" since they behave similarly to nouns in many conjugation patterns. These are contrasted with い-adjectives or "adjectival verbs" which all end with い in the non-past, positive sense and substitute other endings in place of the い when conjugating.