"Fine tuning" is an English idiom which describes the process of making relatively small adjustments to get the optimum performance out of something. In its literal sense, it can refer to a television or a radio that needs to be precisely tuned for maximum effect. More figuratively, "fine tuning" refers to any kind of simple changes that a person makes to get something working or performing just the way he or she wants. Such adjustments are often done to things that aren't quite lost causes but aren't quite performing at a top level either.
There are often occasions when someone uses a phrase or a saying that, if taken literally, would not make sense at all. That's because these phrases are idioms, which tend to take on meanings quite different than either their original intent or the literal meanings of the words they contain. These idioms are useful when speaking with people who are familiar with the expressions. One popular idiom is the phrase "fine tuning."
Whenever this phrase is used, it means that there must be some sort of adjustment being done on something. The key to the meaning of the phrase is that these adjustments are relatively minor, either in terms of scope or in terms of the effort being put forth to make them. If the changes were more substantial in nature and caused a great deal of strain for the person performing them, they likely would exceed the scope of what is considered "fine tuning."
This phrase implies that there isn't much to be done to get things in complete order. For example, someone might say, "I'm not 100 percent happy with my investment portfolio, but all it really needs is a little fine tuning." In this example, the portfolio is not in awful shape, but it could be performing better. Just a few small adjustments will likely get it to where the person in question is completely satisfied.
The origins of the phrase come from the fact that appliances like old-fashioned televisions and radios actually had tuners on them which needed to be adjusted by their users. It's easy to picture someone turning a knob on a radio in tiny increments to try and receive a station's signal just right. There are times when the phrase "fine tuning" may actually still refer to an actual tuner, but, in most cases, it requires a less literal interpretation to understand the meaning of the idiom.