To "attract customers" means to bring more customers to your store or website.
The word "attract" means to pull things closer. The base meaning of the word is to pull closer with a magnet or some other force. You can also use "attract" to talk about love or romance between people:
You're obviously attracted to her.
But when talking about companies, "attract" means to bring more customers:
What can I do to attract new readers to my site?