what does bandwidth mean in oscilloscopes?

Band: a spread of frequencies

Bandwidth: the size of the spread of frequencies.

Since all scopes can measure DC (zero Hz), the bandwidth is simply the highest frequency measurable.

It all depends upon your application. Bandwidth just implies how vast a spread of input frequencies that the device can measure accurately.

A 5 MHz scope will be able to view signals from DC to 5 MHz for sine waves. Other waveforms, you can use it to about 1 MHz.

A 10 MHz scope, just double the above numbers.

So it depends on what you want to look at. Audio only, either one would be fine. AM band RF, you need the 10MHz one. FM band, you need a 500 MHz unit, although a spectrum analyzer might be more useful.

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It's the highest frequency signal that it can display.

An analog 'scope can usually display higher frequencies with reduced accuracy, however digital 'scopes tend to have a hard cut-off and are useless at anything above that.

What will hold a lot will hold a little.