[High School Thermodynamics] I really don't understand reversible vs irreversible processes...

What is the difference?

The one example given to me was that of a piston container with gas in it, piston is being pressurized from outside by some masses (2 different cases).

Case 1: The mass is infinitesimally fine dust.

Case 2: The masses are two finite mass blocks.

So they say, if in case 1 (hypothetical, not possible) you remove dust particles one by one, you are keeping the system and surroundings in thermodynamic equilibrium all along. If you were to put back those dust particles, system and surroundings will be restored to the initial state functions.

In case 2 that last part is not possible (I don't understand why), you remove 1 block, gas expands rapidly due to large pressure difference created, and then settles at another equilibrium point depending on the second mass. Now the part I don't understand is that, why is this case IRREVERSIBLE? Let's say the container was adiabatic, smooth to the last minute detail, so no frictional losses, now if I put that mass 1 back on the piston, the system, and surrounding, should come back to initial states. Which quantity will I find different finally?

Where am I going wrong?

One possible explanation that I thought of was that I had to do a finite amount of work in the second case to bring it back to initial conditions, meanwhile in case 1 the dust particles were infinitesimally light. Does that have something to do with it?