To expand on this because I have nothing better to do, and for the benefit of our younger summoners:
The old rune system the vets and, well, anyone more than about 2 seasons old misses had a truckload of runes you could assemble into pages. There were 4 types, each assembled into a full page by the following amounts:
9 Red*
9 Blue*
9 Yellow*
3 Quintessence
*These had non-color based names and literally no one cared. Even colorblind people like me call them by the colors.
Now, these runes only did one thing: they would increase a stat. You would go to the store and pay blue essence (called IP back then, if you're new enough to not remember the old runes, but it's the same stuff) to unlock a rune. You would then add that rune to whichever pages you owned and wanted it. and then select that page in champ select. You would then receive the corresponding stats in the game. In total, a full 30 rune page had the approximate stat bonus of a partially completed item(around 1500 G if memory serves), though the exact gold value varied quite widely depending on your selections and the game balance at the time.
However, there were a LOT of options. You could accentuate your late game power with runes that started weaker but ended up 2-3 times as strong, or take static runes for a stronger early boost that never grew. each color had an affinity, with reds being offensive, blues doing the best MR and regen, yellows doing the best armor, so you could maximize their individual effectiveness at the cost of focus, or make a strongly focused set at the cost of efficiency. You could stack damage, attack speed, mana regen, health, health regen, move speed, gold gain, %max health, armor, MR, energy regen, and more. It was effectively crafting an item with no passive or active to your personal playstyle, and it didn't take a slot.
These couldn't be changed in the lobby, but they were paired with mastery pages, which could. These mastery pages were similar to the rune pages we have today, but were much larger if a little less exciting on each level (simply more stats in most points). Those pages were free, but pre-making them was advisable since putting one together could take a bit longer than the current iteration.
As someone who had 20 pages, I had 2 for each of my mains at any time(to adapt to enemy team comps and likely strategies), one for "main" champs of off roles when I couldn't support(unusual but they did see use), 4 more for general styles when I was trying someone new but didn't really know what to take, a couple for ARAM, and a couple for troll builds, and a few leftover to change when I wanted to do something specific. Each of these was directly paired with a mastery page so I didn't have to stress out and screw up in champ select.
The major downside was that you had to buy the runes. Want 9 attack runes? cool, get them or get the 6300 BE champ you wanted them for. Oh now you want SCALING attack? pay up again. It gave massive and kind of unfair flexibility advantages to the older crowd, which were only made worse if the younger player couldn't even afford a full page yet.