The Lasker Defense is an excellent way for Black to deal with the leather White throws at him in the opening engagements of the QGD. After 6.e3 you will have reached a very common position in modern competition.
As Black you have a solid, compact position with no weaknesses. The problem is, however, White has all the space and his pieces are much more active. Compare the dark square Bishops for instance. Look at your minor pieces on the Queenside. They compare unfavorably with White's pieces. What to do?
Lasker came up with the solution. Trade as many units as you can. This reduces the value of White's spatial advantage significantly. After 6...h6 7.Bh4 Ne4 8.Bxe7 Qxe7, the dark square Bishops have been traded. White's active piece for Black's passive one, a good trade for Black.
And now you have a well posted Knight. Given your solid structure you can expect to get good chances in the endgame when your Queenside pieces emerge.