What jobu probably means, is that Mars' atmosphere is too thin for parachutes to slow a reasonably sized craft down to low speeds (that's why MSL's landing required additional rocket propulsion [1]). Compare with Earth: Here, the terminal velocity of a reentering capsule is slow enough for parachutes to take over, which in turn slow it down enough for touchdown.

[1] https://en.wikipedia.org/wiki/Mars_Science_Laboratory

It works great for lighter unmanned missions, but becomes less useful as you increase in mass. NASA has had to come up with some interesting solutions in order to land their rovers. Mars Pathfinder used a giant inflatable airbag to scrub the remaining speed it had when hit the martian surface(Lithobraking.) MSL Curiosity, after it jettisoned it's heat shield and parachutes, used a rocket-powered 'sky-crane' in order to slow down the last 200 MPH for a soft touchdown.

This is one of the reason SpaceX is focusing on rocket-powered landings of boosters and capsules vs. parachutes. This is a vital technology required to be able to put large amounts of mass on the surface of Mars