Your "target search phrase" is the search phrase you want to perform well for - so when anyone searches on Google for this phrase, you want to appear as high as possible in the search results.
Your target search phrase will usually be a combination of "what you do" plus "where you are". It is important to include e.g. a town or county if your business serves a local market. Examples of this type of target search phrase are:
If you serve a wider geographic market, it is usually best to be as specific as you can when describing your service. Think of being the best in your niche rather than trying to compete with everyone. For example, if you are a management consultant offering team building services to the retail sector:
The above examples are just illustrations and you should do some research on Google to see how many other businesses are competing for the target search phrases you are considering. This can help you fine-tune your target search phrase until you're happy it's specific enough for you to stand a chance of competing but not so specific that no-one will be searching!