potential customer is a person who may buy a particular thing. there is a high possibility of this person buying that thing.
coming to the definition of the market,
' potential customers sharing a particular need or want' - those people who have a common need or want for a particular thing and are considered as potential customers because they may buy that
'need or want' - need is your necessity, and want is your choice.
For example, food is your need, and sandwich is your want.
Sandwich is your need, and a particular brand is your want.
'who might be willing and able to engage in exchange to satisfy that need or want' - those potential cutomers may be willing to buy and can buy that particular thing
'engage in exchange' - buy; give money and get something; transaction
A market is a group of those people who have a need for that particular thing and may buy that thing in future.