In the image above, the little plus sign next to the striped egg signifies it's a Plus Egg. It gives bonus points to one of the three stats: Attack, HP, Recovery.
The more points the better. So for instance, if I get two points in attack, then it's a total of +20 (but only for attack).
A total of 99 points for each stat can be administered to a monster card, making it a maximum of 297 points (99 x 3 since there are three stats), no higher. In the end, this serves as a boost to your monster cards, because it greatly increases their stats, making them have more health, deal more damage, and recover more health when they lose it.
How to get them? Well... this is a lengthy process. You can get them as monster drops in dungeons, but only specific ones. And they only come with one extra point per stat, but there's never a guarantee that the drop will be a Plus Egg. Luckily, there are dungeons that always guarantee Plus Egg drops like, Star Treasure Thieves' Den and Ruins of the Star Vault. From time to time, there are in-game events that increase the drop rate of Plus Eggs by 10 times in Special Dungeons.