I want to install ceramic tile. Someone said the preparation and base material are critical to a successful job. Can you explain?

Tile is a great product and when installed properly will last a very long time. It costs a bit more to install but the initial investment will likely out last other softer products many times over. As you suggested, the base is critical to installing tile. I am not an expert but I believe the adhesive or cement product that holds the tile down must have a proper surface to grip to. If you have an OSB subfloor the moisture in the cement can be drawn out of the adhesive into the substrate causing it to fail. Plywood on the other hand is better as it does not absorb as much of the water in the cement. A second factor is the thickness of the subfloor. As you know the tile will not flex so if the floor below the tile is flexible you will soon have loose and cracked tiles. I believe it is recommended that the floor be 1” thick. Contact a flooring contractor for more tips before starting you will be glad you did.