Those long silvery wires on a guitar or a bass are called "strings"; don't ask me why. They've never been made of string. In the old days they were made of gut (that's intestine). On modern classical guitars they're usually made of nylon. On a real guitar -- an electric or a steel-string -- they're generally made of a nickel-steel alloy, sometimes coated with bronze if they're meant for an acoustic.
When you put a new set of strings on a guitar, they'll need to stretch out a bit until they reach a sort of gently wandering equilibrium. You can hurry that along by pulling each string away from the guitar's body several times when you put them on. If you pull too hard on the lighter strings, you may break them, so be careful. Even after they've been stretched in a bit and the pitch is stable, they're still new. They sound bright and lively, maybe a little metallic. They need a day or two to really settle in, if you ask me. Later, after a month or two, they'll sound muddy and dull. I guess it's called "metal fatigue". They won't look so shiny and they'll no longer cut through very well. They're "dead strings". Depending on what sound you want, it might be time to replace them.
All of the physical phenomena here hold true for basses, but a bass is not a guitar. A bass is a bass, and you play basslines on it. The last goddamn thing you want is a bright, metallic sound that cuts through the snare. To hell with the snare! If you want to make noise at all in that frequency range, quit fighting physics and go buy a guitar. Thin, bright, metallic bass tones sound great in the music store but in a band they vanish completely. I've seen god knows how many of these people on stage with fabulous high-tech bass rigs, exotic basses, and shiny new strings, and boy do they sound great tuning up! When you see that, listen hard while he's tuning up and enjoy it while you can, because that's the last you'll hear of him all evening.
Bass strings take a few months to settle in and sound right. They've got to be well and truly dead before they're worth hearing at all. That's just the way it is.