What is the meaning of this small symbol on my mod card?
Solution 1:
That icon means it's an Exilus Mod, which can fit in an extra slot on your warframe (top right) if you unlock it with an Exilus Adapter.
They're all utility mods so you get an extra slot for them, they also fit in regular slots.
A different icon in the same place can mean it's an Aura or a Stance mod, both of which go in the top left slot of your warframe or your melee weapon respectively. Aura and Stance mod add mod capacity instead of taking it so they're always good to use.