Each transaction in Bitcoin consists of inputs and outputs; inputs point to outputs of some past transactions, and outputs become new inputs of some future transactions. Transactions that are connected through inputs and outputs form a kind of chain, which may tell an observer how coins moved, including their possible source and destination. The point of mixing is to break (or at least obfuscate) links in this chain of transactions, making it harder for an observer to determine what entities actually participated in these transactions. The bitcoin blockchain is by default transparent and pseudonymous, and mixing doesn't make it less transparent, but it tries to make it even more anonymous by breaking deterministic links between inputs and outputs. If your goal is to prevent your enemies or envious acquaintances from spying on your transactions and infringing on your private life, then you should consider using mixing tools but choose only reputable ones because with others you may not achieve what you want.