This expression is part of another expression that goes like this:
"You can talk the talk, but can you walk the walk?"
"Talking the talk" is sort of what it sounds like -- you can talk about doing something, you can make it sound good.
"Walking the walk" is actually DOING what you've said. It's giving your words real meaning by acting on them.
Because so many people know the full expression, you will hear people abbreviate it to one part or the other. So, if you hear someone say, "He only talks the talk," that means the person is good at talking about what he should do, but maybe not so good at following through. If you hear someone say, "Can you walk the walk?" it means can you deliver one what you say? Can you do the things you say you can do?
I might use this phrase at my job when I'm working with some other people on a project. Bill, one of the people on the team, says, "I'll have my part done by Wednesday." I know he is being unrealistic, so I say, "Yeah, you can talk the talk, Bill, but are you going to walk the walk?" In other words, I'm questioning whether he'll do what he says.
Let's say I'm thinking about hiring Linda, but her resume seems really long and padded. I call Linda's past supervisor to see what kind of worker she is, and he says, "Linda can talk the talk AND walk the walk." That would tell me that her resume is long for a reason.