This determines the highest frequency signal you can measure. For general hobbyist work, aim for at least 50–100 MHz .
Professional refurbished units from manufacturers like Keysight are calibrated and often come with a warranty. Essential Pre-Purchase Tests buy used oscilloscope
Connect a probe to every channel and attach it to the built-in probe compensation output (usually a small metal tab on the front). You should see a clean, steady square wave. This determines the highest frequency signal you can measure