In addition to what everyone else has said...
Someone could explain what thas it´s mean.
On a performance note it means that you're going to get an index scan instead of a seek.
"I cant stress enough the importance of switching from a sequential files mindset to set-based thinking. After you make the switch, you can spend your time tuning and optimizing your queries instead of maintaining lengthy, poor-performing code."-- Itzik Ben-Gan 2001