An online presence (when used during job application talk) means having social accounts tied to you and more specifically your name
a bad social presence such as posting racial slurs, or expressing bad behavior e.g. pictures of parties, cussing, can affect your job application when reviewed
if you are not being reviewed to be hired it can affect the brand's image
example - you work at a law firm, and you have a twitter account with your full name, the place you work at and you start tweeting how you hate a minority and express your beliefs. when the company sees these things they will not be happy and may go as far as to fire you from said position.
