Visibility

Buy orders have a 'range'. If the range is 'station', only people at the station where the buy order is located can sell to that buy order. If the range is larger, say 5 jumps, anyone within 5 jumps of the station with the buy order can sell to it.

If you set a normal buy order (not a remote one) you can set whatever range you like (up to regional). If you have trained the skill Procurement you can place remote buy orders, but unlike normal buy orders, these always have the range 'station'.

Visibility allows you the set the range of a remote buy order, so instead of the default 'station' the range can be set to 'system' (Visibility level 1), '5 jumps' (level 2), '10 jumps' (level 3), '20 jumps' (level 4) or 'regional' (level 5). Generally it not worth training to level 5 as 20 jumps will cover most, if not all, of any region.