Weight KB: All characters have an internal weight value that comes into effect with weight-dependent throws(i.e., Falcons DThrow, etc.), and also take effect when the game calculates KB. There are weight groups, however, meaning that not each character has a unique weight. READ: WEIGHT != FALLING SPEED Weight is a character's "resistance" to knockback, and so in weight KB weight is taken into account. So like, Lucario and Ivysaur both have a weight value of 1, so they are what you start with when calculating throw frame data. They also both take the same amount of knockback in duplicate environments. As opposed to, say Bowser, who has a higher weight value, and so take Falcon longer to throw, or Jigglypuff, who has a lower weight value, and who is thrown quicker.
Base is basically the KB dealt at 0% on someone with a weight modifier of 1. It can also apply to set KB, like in the weak hit of ROB's UTilt. I think.
Growth is simply increase in KB relative to opponent %.