Solo Instances are instanced zones which a player cannot enter in a group. As with all instances, mobs do not respawn when killed, and there usually is a good amount of XP and drops to be had. Solo instances may have darkblue/repeatable quests, which are a good way to level up. Although the player cannot be in a group to enter a Solo Instance, he/she can group with other players once inside and still get all drops/XP.