Let's first go over the formulas for Molality(m) and Molarity(M).
m = moles of solute/kg of solvent
M = moles of solute/ L of solution
The main difference is in the denominator, while Molality has kg of solvent in the denominator, Molarity has liters of solution in the denominator. Let's remind ourselves that solute is what gets dissolved, solvent is what dissolves it (a good example is water) and solution is both solute and solvent together.
For example 20g of NaCl are dissolved in 100ml of water. Thus, HCl is the solute, water is the solvent, and together they make a solution.
For more help with chemistry on all levels, please contact Transformation Tutoring, and our amazing chemistry tutors in NYC, Brooklyn, and online will be happy to help you!