Diagnosing water heater error - Need help with diagram
The slash indicates that the schematic line indicates a bus of five wires, rather than just a single wire. For the /5 example you call out, it means that all five pins on the X2 connector of item 1 are connected to the corresponding five pins of the X2 connector (item 13). If connector X2 is implicated in a troubleshooting guide, you may want to use a multimeter to ensure that all five pins of the cable still have continuity from one end to the other, and there are no short circuits between adjacent pins. This is in addition to any further measurements (e.g. of voltages on those pins) that the manual recommends.
Likewise, item 2 presumably has a single three-pin cable going to the three-pin connector X10 (item 12).
The /1 going to item 3 is a bit of an oddball - there's a single wire, and it's specifically called out as a single wire. The return for that signal might be chassis ground (given the dotted lines between that wire and ground on/near the X7 connector).