Phishing links are links which fraudsters use to trick people into providing the access credentials to online accounts. If you click such a link, they can then use your personal information to commit identity theft, charge your credit cards, empty your bank accounts, read your email, and lock you out of your online account by changing your password.
Phishing links are usually sent in phishing emails and often lead to a phishing website.
Since phishing links are rigorously filtered out by email clients you should make sure that nothing in your email content could be mistaken for anything malicious. This page will help you avoid the most obvious pitfalls.