Warning: This answer has spoilers for the end credit scene in "Big Hero 6."
To answer your question, it shows Fred at the end inside his mansion, while staring at a family portrait of him and his dad together. As you probably saw earlier in the film, the father looks remarkably like Stan Lee in the portrait. Anyway, Fred says how he wishes he could share his accomplishments with his father, and how he's always felt a great distance with him being away on the family island all the time.
As Fred says this part, he places his hand on the wall that uncovers a secret door. It opens up to a room where it looks like a secret superhero hideout, with colorful blue costumes hanging on the wall. Fred looks around the room, and then hears a voice calling his name. He turns around.
It's none other than Stan Lee himself, as he says, "Son." Fred replies, "Dad?" Then Stan Lee picks up a pair of blue briefs with two red dots on each side. He holds it up and says, "I wear them front." He turns the underwear around and says, "I wear them back." Fred joins him as they say the next lines together, while Fred has a huge smile on his face. They both say, "I go inside out then I go front and back." Afterwards, Fred says, "Dad", and then hugs his father.
Stan Lee says, "Fred, we have a lot to talk about." The end.