Gravimetric analysis refers to any method for quantifying the amount of an analyte based on mass. For analytes dissolved in a solution, this may require first reacting the analyte with another species to cause it to precipitate out of the solution. The solid product can then be filtered and weighed, and the mass of the analyte initially dissolved in the solution can be determined. Other situations may require different methods of preparing the analyte in an appropriate form for weighing, but the general idea is always that the composition of the weighed species is known, so it’s possible to figure out the mass of the analyte originally present.