What does this mean and does/canit have any negative effect on my truck?

My mechanic said he thought the truck was a bit sluggish so he "advanced the timing 3 degree's". He said this should improve milage and also make it a bit more responsive. So far I guess it does seem a little more snappy around town.

My only question is though, will this possibly cause any other issues with the engine?

 

adjusting the ignition timing has to do with the delay between when your piston is at top dead center (up as far as it will go in the cylinder) and when your spark plug for that cylinder fires.

will it hurt your vehicle?
probably not, but it depends on where it was set to begin with. if it was retarded below the factory setting and he set it up three degrees back to where it should be, then its fine.

eventually, i guess if you adjust it too far you could get spark knock or it just wouldnt run at all i guess.

 

you can safely do it a few degrees from stock, my stock setting is 8* i think and run it close to 15*