Mixed (or co-ed) sex slow pitch softball is a social team sport, played by teams made up of 5 men and 5 women. The ball is thrown (pitched) underarm by a player called a pitcher and, hopefully, hit by the batter and "fielded" by players on the defensive team who wear "gloves" on their non-throwing hand to catch the ball. Its closest comparable sport is baseball but softball is much more popular in the UK.
Heres a quick run-down of the main rules:
The playing field, or diamond
The field is made up of three bases (first, second and third) at the corners of the infield, plus a home plate over which the batter tries to hit the ball. The four corners make up a square, on end, or a diamond shape.
Scoring a run
A run is scored every time someone from the batting (offensive) team completes a circuit, having touched all four bases in turn. It doesn't matter if the batter-runner has stopped at each base, while others come up to bat, or if s/he manages to run the circuit in one go (that's a home run!)
Running around the bases
Batter-runners, standing on a base and waiting for a chance to run further as another team mate takes a turn at bat, are not allowed to leave their base until the ball is hit by the batter. (Unlike baseball or fastpitch softball, you're not allowed to steal bases.)
How to get someone out
Batters can be caught out by fielders, if they catch the batted ball before it hits the ground. Batter-runners can also be put out by tagging them (touching them with the ball) if they are not touching a base at the time. If a batter-runner has to get to a base, they don't need to be tagged to be put out: a fielder can simply hold the ball and put their foot on the base before the runner gets there (that's a force out). Another way to get out when batting is to fail to hit the ball into the field, after three good pitches (strikes): this is called a strikeout. Mind you, if the pitcher throws four poor pitches (these are called "balls") at the batter, the batter gets a free walk to first base.
After three outs, the teams change places and the fielding team then gets to bat. The same person can be out twice: if they bat once, and get out, it may be possible for the rest of the team to all bat and the person who was out previously bats again. After both teams have batted, that is called an innings. There are usually seven innings in a game, but can be more if the match is tied at the end of seven innings.