Those are references to the monster infusion system. Sacrificing a monster crystal to infusion removes the monster from your inventory, but bestows the abilities that monster had into a new one.
Every monster has 10 passive ability slots: when you get a monster crystal, some of these slots are taken up by abilities. If you look at a monster's ability list, you may see a red lock, a yellow lock, or no lock at all next to some of the abilities.
Because I was confusing myself as I was writing this, I'm going to talk about locking from two perspectives separately: what it means for sacrificing a crystal and what it means for infusing a monster.
Sacrificing a monster crystal
- Red-locked abilities on a crystal are not infusable: you can't pass these off to a new monster during infusion.
- Unlocked and yellow-locked abilities are infusable and can be transferred during infusion.
Infusing a monster with a crystal
When you infuse a monster that already has its passive ability slots filled, some of those filled slots will be replaced with the abilities from the infusion based on the following rules:
- Unlocked abilities can be replaced through infusion: lower ranked abilities are replaced first.
- Red-locked abilities can never be replaced: they essentially cut the number of free slots a monster has for customization. Either the monster starts with them or acquires them through the Crystarium.
- If you repeatedly infuse a monster with the same ability (like, say, +N to magic), you can "yellow-lock" that ability, which effectively prevents it from being replaced during future infusions.
Essentially, the main difference between red-locking and yellow-locking is that yellow-locked abilities are viral: they can't be replaced,1 but they can be transferred to new monsters through infusion.
Note 1: to a point; there are certain things you can do to remove a yellow-locked ability. See the Yellow Lock Rules in BMSirus's Monster Infusion FAQ for what those are.