A platform feeder is any flat, raised surface onto which bird food is spread. They are open in design and the birds can easily keep a sharp eye out for any approaching predators and if any are spotted the birds can easily fly away. Trays attract most species of feeder birds, but without a baffle, they offer no protection against squirrels or chipmunks. If you place a tray feeder (with legs) near the ground you'll most likely attract juncos, doves, sparrows and cardinals.
Some tray/platform feeders are designed to be placed on a pole or post underneath your hopper or fly-thru feeder. Their purpose is two-fold: to provide another area to place food and also to collect any food from the feeder above that has been scattered.
A platform feeder with a roof, also known as a fly-thru, has a roof that will help keep the seed dry but no container to hold seed. These feeders normally don't hold as much seed as a hopper and need to be filled more often. They should have plenty of drainage holes to prevent water accumulation. Fly-thrus can be hung or mounted on a pole or post.