Multiple choice
A multiple choice field allows your contact to indicate several variables that can be combined. Example: the field is "favourite dishes", with "pizza", "burger", "salad", "pokebowl" as choices
As the field is multiple choice, I can select several answers: "pizza" AND "burger".
Drop-down list
A field with a drop-down list allows you to make only one choice among several proposals. e.g.: the function is generally a drop-down field => I cannot declare that I am a Butcher & a Developer in the same company, I must choose a value.