A preamp is a device that takes your guitars weak electrical signal, and converts into a signal strong enough for further processing, EQ-ing etc., and for sending to a power amp and loudspeaker. Most guitar amps have a preamp stage built in. Many preamp devices let you shape your tone in unique ways, separate to or instead of your amp.

Preamps convert your guitar's weak electrical signal into a signal strong enough for further processing. Many guitar preamps also incorporate more effects and tone-shaping capabilities. Most guitar amps have a preamp stage built in.

Do I really need a preamp?

At some point in your signal chain, yes! The preamp converts your guitar's weak electrical signal to one strong enough for further processing, so you do need it. Many guitar specific preamp devices have other fantastic effects, EQ settings and tone shaping capabilities built in too.