This can range from a cautious interest to early strong desire, such as when a woman sees a man she does not know at a party and is immediately attracted to him.
Knowing about them may happen in various ways, for example a man may see a woman in a bar or a sales person hears of a possible customer from a colleague.
There is also information exchange which helps with the next stage of deciding where to take the relationship.
A typical question to help this is 'What do you do?
If there is a third person helping out, they may volunteer information, for example where a friend is 'match-making' or a company researches prospects for a salesperson.
With enough information, the motivation for a relationship begins.