In general play a player is offside if the player is in front of a team-mate who is carrying the ball, or in front of a team-mate who last played the ball.
Offside means that a player is temporarily out of the game.

OFFSIDE AT THE SCRUM

When the scrum is set, the scrum half not throwing the ball into the scrum must take up a position either at the same side of the scrum as the scrum half throwing in the ball or behind the offside line defined for other players.

When a team has won the ball in a scrum, the scrum half of that team is offside if both feet are in front of the ball while it is still in the scrum. If the scrum half has only one foot in front of the ball, the scrum half is not offside.

When a team has won the ball in a scrum, the scrum half of the opposing team is offside if that scrum half steps in front of the ball with either foot while the ball is still in the scrum.

The scrum half whose team does not win possession of the ball must not move to the opposite side of the scrum and overstep the offside line. For that scrum half that runs through the hindmost foot of that player’s team in the scrum.

The scrum half whose team does not win possession of the ball must not move away from the scrum and then remain in front of the offside line. For that scrum half that runs through the hindmost foot of that player’s team in the scrum.

Offside for players not in the scrum. Players who are not in the scrum and who are not the team’s scrum half, are offside if they remain in front of their offside line or overstep the offside line which is a line parallel to the goal lines and 5 metres behind the hindmost player of each team in a scrum.

If the hindmost foot of a team is on or behind that team’s goal line, the offside line for scrum halves and non-participants is the goal line.

OFFSIDE AT THE MAUL and RUCK

There are two offside lines parallel to the goal lines, one for each team. Each offside line runs through the hindmost foot of the hindmost player in the maul. If the hindmost foot of the hindmost player is on or behind the goal line, the offside line for the defending team is the goal line.

A player must either join a maul, or retire behind the offside line immediately. If a player loiters at the side of a maul, the player is offside.

Players joining a maul must do so from behind the foot of the hindmost team-mate in the maul. The player may join alongside this player. If the player joins the maul from the opponents’ side, or in front of the hindmost team-mate, the player is offside.

All players in front of the offside line and who do not join the maul, must retire behind the offside line at once. A player who does not do so, is offside. If any player who is behind the offside line oversteps it and does not join the maul, the player is offside.

Players who leave a maul must immediately retire behind the offside line, otherwise, they are offside. If the player rejoins the maul in front of
the hindmost team-mate in the maul, they are offside. The player may rejoin the maul alongside the hindmost team-mate.

When players of the team who are not in possession of the ball in the maul voluntarily leave the maul such that there are no players of that team left in the maul, the maul may continue and there are two offside lines. The offside line for the team in possession runs through the hindmost foot of the hindmost player in the maul and for the team not in possession it is a line that runs through the foremost foot of the foremost player of the team in possession at the maul.

When players of the team who are not in possession of the ball in the maul voluntarily leave the maul such that there are no players of that team left in the maul, players of that team may rejoin the maul providing that the first player binds on the foremost player of the team in possession of the ball.