In the video you posted you can hear the referee shouting "just a tackle". In doing so he's telling the players that no ruck has been formed and hence there is no resulting offside line. (See Law 16)
A ruck is a phase of play where one or more players from each team, who are on their feet, in physical contact, close around the ball on the ground. Open play has ended.
However, there has been a tackle so, at that point, play is now governed by Law 15. This says that, after a tackle, players not involved in the tackle itself can only enter the tackle area though what is known "the gate". The term 'gate' is not defined in the laws but it is effectively shorthand for the following text in laws 15.6 (c), (d) & (g).
These say that any player getting involved in play in the tackle area must do so ...
... from behind the ball and from directly behind the tackled player or the tackler closest to those players’ goal line.
This is much easier to understand with a picture so have a look at last image 'the tackle gate' on World Rugby's page that explains the tackle.
The referee's comment about 1 meter is him explaining that if a player approaches close to the tackle area but does not do so through the appropriate 'gate' then they will breach part of Law 15.6. (Note that the laws do not mention 1m specifically, instead it's just a pragmatic rule of thumb for defining the tackle area.)