This guide is a good way of playing the champion, Jarvan IV. It provides a good off-tankiness but also keeps the damage you deal fairly high. I've done this because it's boring to just go tanky and deal no damage. Yes, you're a valuable asset to the team when very tanky, but if you can be tanky and also deal good damage, you're an even better asset to the team. The tankiness enhances his ulti through making him strong enough to hold in multiple opponents for the duration of the ulti, so all of your teammates on the outside of the ulti can range the opponents inside the terraformed arena.

These runes are tanky, but also give useful attributes towards attacking opponents. The armor and magic resist give tankiness. The armour penetration giving the attacking attributes. With the mixed quintessances it gives good boosts on more than one attribute for battles.
The flat armour runes are good for your solo top because they give you a defensive boost right from the start of the game giving you an upper hand in a early fight. The same goes for the magic resist runes. But because the armour and magic resist runes are fairly high, they will help get through the whole game as well.

The same goes with the masteries than what goes for the runes. Some tankiness and some attacking attributes.
The lifesteal mastery points help you to be able to lane for longer making it even better for if you're playing solo top.

All of the items contribute to tankiness (armour, magic resist and health). The Tiamat provides decent AD but also HP and Mana regen with it. The splash damage in the passive makes it especially good for farming minions. By the end of the build, the HP regen is so high that it will make a huge difference in team battles even if you're being attacked.
I've tried to start off with the fairly cheaper items so you're not taking ages to get them nd you don't fall behind.