I don't understand the difference in the thought of phenomenology between Husserl and Mearleau-Ponty.

Phenomenology is a forbiddingly complex subject, so let's try and keep things as simple as possible. Husserl's phenomenology, in the broadest possible terms, is Cartesian. What this means is that the focus of Husserl's approach is on the mind, on human consciousness. Other considerations are bracketed-out, or put to one side, as being irrelevant to Husserl's investigations.
This means that human beings, as embodied in a social world, a world of other selves, are ultimately of secondary importance to Husserl. The picture that Husserl gives us of persons is, therefore, somewhat one-dimensional. In putting to one side our bodies, our world and our interaction with others, he is excluding all those important things that make us fully human. As such, he is presenting us with a narrow understanding of who and what we truly are.
Merleau-Ponty's approach seeks to remedy the deficiencies of Husserl's phenomenology in relation to the body. In common with other phenomenological thinkers such as Heidegger, he believes that it's the human person as he or she exists in the everyday world that should be the focus of phenomenological investigation. Husserl, on the other hand, is concerned with the contents of the disembodied consciousness and how they present themselves.
Merleau-Ponty, unlike Husserl, does not believe that the human body can be separated from any meaningful phenomenological understanding. It's our bodies that ground us in the world, pointing towards the existence of other selves and objects in space. Husserl wants to put considerations of body to one side; yet Merleau-Ponty sees phenomenology as only being possible in the first place precisely because of our physicality and how our bodies locate us in the world of space and time.
Our bodies, and all they represent--sexuality, language, emotions--are the true locations of meaning, providing us with a pre-cognitive understanding of the world before our active minds get to work. And it's the latter that remains the focus of Husserl's phenomenological investigations.
https://plato.stanford.edu/entries/phenomenology/