BLOCKING AND SILENCING
If the admins of Instance1 BLOCK Instance2, the users of Instance1 cannot interact the users of Instance2
If the admins of Instance1 SILENCE Instance2, the users of Instance1 can still privately follow and interact the users of Instance2, but those interactions (like boosts) will not be seen by the other users of Instance1
To be more clear: BLOCKING is used for instances that permits horrible things and behaviours and SILENCING is used for instances that your Instance tolerate but doesn't appreciate so much.
The same things are appliable to users.
Also you, as simple user, can decide to block or silence (for your eyes only) other users and entire Instances.