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From a practical perspective, the game is played 11-on-11 where there is 1 goalkeeper and 10 field players per team. These field players tend to be divided between defenders, midfielders and forwards. The positioning is dynamic so players will vary between defending and attacking frequently.

Rules-wise, the ball can’t touch a player’s hands unless it is the designated goalkeeper in the goalkeeper box. A foul (eg dangerous tackle) or handball or other infraction can lead to a free kick or a penalty kick if it occurs in the goalkeeper box. Infractions are judged by the main referee for explicit rule-breaking but also perceived intent which could lead to a verbal warning, a free kick, a yellow card (formal warning; max 1 per game, 2nd yellow card = automatic red card) or a red card (immediate expulsion and team plays remainder of game less one player).

For out of bounds, only the ball can ever be out of bounds and, therefore, out of play. Players can cross lines but it’s only the ball that matters and crossing lines requires the entirety of the ball to cross them (unlike tennis). Sideline (touchline) outs are put back into play with throw-ins while goal line (where the goals are) outs are put back into play with a goal kick.

There are various rules about when a ball or a player is in or out of play for offsides but basically an attacking player cannot be ahead of the second-to-last defender before the ball is passed to them. The position of the ball itself does not enable an attacking player to legally be offsides but rather the moment it leaves the foot of the passing player while the attacking player (target of pass) is onsides.

Theoretically, possession (who has the ball) tends to be very important because it sets the pace of the game and usually the team that doesn’t have the ball doesn’t score.

There’s a lot more but this is the baseline basics.