This is a great question! I know so many people that have taken circuits and physics courses and lack the actual understanding of what is happening. There are a bunch of mathematical proofs you can look up (highly recommend), but I will keep it short, simple, and strictly conceptual since I do not know where you are academically.
The scenario is that you have a circuit that has a few different resistances, like the one shown below. V is the voltage source, and R1, R2, and R3 are resistors. There is a current, I, flowing through the circuit that will meet each of these resistances.
The main key is to understand what voltage is. Voltage is just a way to describe how much energy is supplied from a source. You can kind of think of it as an electric potential energy.
This energy is traveling in the form of current and meets disturbances along it’s path, called resistances. The larger the resistance, the more energy it takes to get through that resistance, and as a result, it loses that energy which is dissipated as heat. So, after every resistance the current goes through, it loses energy through heat dissipation, and thus experiences a drop in energy/voltage. The larger the resistance, the larger the amount of energy it needs to use in order to get through.
A voltage drop just describes how energy is reduced as it goes through a resistance.
I hope this helps give you a simple conceptual idea of what a voltage drop is!