Have you ever had a lettuce and it was slightly bitter?
It's been "bolting".
No, the CC is not making this term up. It seems to be standard terminology in the horticultural world.
Bolting is the plants' attempt to produce seeds, flower and reproduce.
Once plants have bolted, there's no going back because it's just a fact that they just want to flower and reproduce whereas we want to eat them. They turn bitter because it's a first-line defense against predators including humans who most certainly are exactly that.
There's a general tendency of humans to buy larger produce. More bang for the unit buck and all that but you'd be better served to pick smaller varieties that haven't yet bolted out of town.