Yes, that's right: it's just a line: so not very solid at all.
Nowadays we tend to call a solidus a forward slash, but this not only takes a little longer to say and write but is confusing because it doesn't really go actually forward at all.
In fact, I would say that a solidus slopes backwards, myself.
Just to make things even more complicated, the backwards slash slopes forward:
(If you're being technical, that's a reverse solidus, but that doesn't help matters at all.)
Perhaps we should have called the solidus a standard slash, or just a slash. It would have surely been easier for everyone, but, hey, it's almost certainly too late, now.
Solidi can be seen all over the place: in abbreviations (w/e (week ending)) in fractions (14/24, which means fourteen twenty fourths, which is the same thing as fourteen divided by twenty four) and of course in dates (DD/MM/YYYY). Or, if you're in the States, MM/DD/YYYY.
A solidus will often also mean or, as in M/F (male or female) or Y/N (yes or no).
In fact they're useful beasts altogether.
Pity about the name, though.
Thing To Use Today: a solidus. A solidus used to be a Byzantine gold coin: solidus meant solid, which is of course what you want your coins to be. By Medieval times a solidus meant a shilling, and was indicated by a long s, which eventually stretched out into our slash/solidus.