Helmets, Catcher's Gear and Cleats
The safety equipment, plus the one rule about it that decides whether a player can take the field at all.
What it actually is. Every hitter and baserunner wears a batting helmet, with a double ear flap required at every level below the majors. A catcher wears a helmet and mask, chest protector, shin guards and a cup. Cleats come in metal and molded plastic, and which is allowed depends entirely on the sanction and the surface.
What somebody who knows the game gets out of it. The metal spike question is the one that actually catches families out, because the answer changes by age, by sanction and by field. Most youth sanctions prohibit metal below a certain age, many turf facilities prohibit them for anybody regardless of age because of surface damage, and some tournaments allow them only for specific divisions. It is not a safety judgment made once, it is a rule that has to be checked per event, which is why this site fetches the current rules rather than storing an answer.
The numbers worth knowing. Metal is typically permitted from around 13U or 14U upward in most sanctions, and prohibited on many turf fields at all ages. Catcher's chest protectors are sized in inches from the neck to the waist.
What people get wrong about it. A catcher's helmet and mask must be worn by anybody warming up a pitcher, including between innings and including a coach, at nearly every level. It is the most commonly ignored rule in youth baseball and it is in the book precisely because it is where injuries happen.
At Little League and travel ball. Check the tournament rules before the weekend, not at the gate. Metal spikes, bat stamps and helmet requirements are the three things that get players turned away at check in, and all three are answerable in advance. The travel section of this site covers all three by sanction.
Batter's Eye: look up anything in baseball