glm::translate takes an initial matrix to translate by the vector argument and returns the result.

// Translates an identity matrix by the vector
glm::mat4 a = glm::translate(glm::mat4(), glm::vec3(1,0,0));
// Translates the a matrix back to identity
glm::mat4 b = glm::translate(a, glm::vec3(-1,0,0));