Grooming in essence is gaining the trust of a child (child here including teenagers), and conditioning them to see predatory behavior as normal.
The predator focus special attention on them, forming a relationship and making them think they are special/important in the predator’s eyes.
They’ll give them special consideration above any other child (students, daycaremates, sons/daughters/cousins, etc), give them attention and affection, make the child think they’re really wanted.
This bond of trust is really important because when it comes time for more intimate advances the child will think “i think this is weird, but it must be okay because this adult is trustworthy”.
The child is now percieving a sexual relationship with this adult as something they want, and sexualization of themself as part of the special attention that they genuinely like and desire.
If you’re on the lookout for someone who may be grooming your child, look for adults who highly favor your child, and that your child may rave about because they like them so much.
This could be perfectly normal, because many predators are extremely smart about the way the approach a sexual relationship with minors, but if you look close they may be getting VERY close with your child, showing small signs of seemingly innocent (potentially frequent) physical contact.
Also, if the adult is someone your child frequently talks about and is excited about seeing or talking to, then suddenly fails to bring them up at all, that could be a sign that an instance of abuse has occurred.
Again, all the signs of grooming can be very subtle and difficult to detect, and the abuser can be and often is someone youself trust, like your sibling, or a teacher.
Always be cautious when it comes to other adults interacting with your child in a place separate from you.