Sartre assumes the lack of God, and takes this to entail a lack of meaning to the world a priori. That is, to Sartre, if there is no God to determine the meaning of things--the mattering of things to persons--then it is up to whatever mind there is to determine how things matter.
The dependency of meaning on mind is right. To matter, a thing must matter to something. That meaning is undetermined apart from a prior mind setting the meaning seems, to me, to be wrong. To set the meaning of words involves a cultural use of the words in a certain way, and this is a case where meaning is set by minds in a wholly contingent way. On the other hand, that breaking my leg is painful is given by the facts of how the world is and how I am, and it is such that the relevant aspects of my makeup are not (easily) available for me to change. Thus, the meaning of a broken leg as bad is given by how things are, independently of the choices of minds.
Where God exists, he may set the meaning of things by setting how the world is. He creates a material world with meaning to us by creating it and us in a particular way. The matter of creation coheres, in this case, with the will and command of God.
Sartre's view of how things mean is dominant in today's society. The idea that an action may mean "nothing" depends, in many cases, on the assumption that the human will is in charge of the meaning of things, of bodily actions. The thought that music or sex might not be taken seriously as meaning what they appear to mean, or have historically meant, depends on this view.
An alternative materialist view would be one which said that where only matter exists, the structure of matter is what determines the meaning of life. This would make the meaning of certain bodily actions (sex, cutting, drugs and alcohol, etc.,) dependent upon how those actions affected one and those around one.
The Sartrean view holds, also, that we are ultimately free. That is, that we may do whatever it is we choose to do. This does not mean we can fly, but that we may seek any aim we will. It is not a denial of the material structure of the world's impingement on what and how we can act, but a denial of the idea that we are constrained by our character or upbringing. It is a denial of the fittingness of allowing a pre-existing culture to impact one's own behaviors. Sartre was hipster before it was cool.