Terminology used by players.
This is a common term used by players to refer to the state where you can no longer progress on the map, be it because everyone is frozen, or you can't do a part alone and your partners are not in a place you can reach.
We refer to this term as map parts where you can fail.
A blocker is a player who intentionally fails or kills other people.
If you see someone blocking you can report him on the discord server in the #reports channel.
These are servers based on the DDNet mod where you can block freely.