An O-ring in a plumbing or hose fitting is a doughnut-shaped piece of rubber. It creates a seal between the pieces of a fitting to prevent water leaks.
The earliest sign of O-ring wear is usually a slow drip from the affected fitting. As the O-ring breaks down further, the slow drip becomes a faster drip, and can become steady spray of water out of the fitting.
The solution is to replace the worn O-ring. Disconnect the hose and remove the worn O-ring from the fitting. Remove the old O-ring and insert a new replacement, available at most hardware or plumbing supply stores.