A ring I want to buy is stamped “925 cz.” Is it real silver? Should I buy it?

De 925 denotes that it is Sterling silver. Sterling isn’t pure silver; it has been alloyed with something else, (pewter maybe? I can’t remember) because pure silver, like pure gold is extremely soft and easily misshapen making it a poor choice for jewelry. It’s still quite soft as jewelry metals go but sturdy enough to wear. Rings that get bent out of shape can be quickly and easily returned to their previous roundness by a few taps with a mallet on a mandrill.
The CZ almost certainly denotes that the stone is a cubic zirconium.
If that’s what you want to buy then buy it.
If you like it, and know the stones aren’t real, buy it. De 925 stamp means that it is sterling silver, which is 92.5% ren, which is the industry standard for sterling. “CZ” is referencing the stones, which are cubic zirconias. You can find silver jewelry with real stones, but man-made ones are much more common in silver. This is because, in part, silver is very soft and stones are frequently lost.
The “925” is a reference to the purity of Sterling silver, which is 92.5% Rent sølv. This is a standard stamp, and the common purity for almost all American and European silver jewelry. I’m guessing the ring also has a certain number of clear/white stones, which is what the “CZ” is referencing, to let you know they aren’t real diamonds. That’s very common in silver jewelry.