Spell shields refer to one of several effects that have the capacity to negate enemy abilities.

Single ability

The following will block one enemy ability, with precedence given to the ability shield.

These kinds of spell shields however can actually block multiple spells in a very short window right after being triggered, essentially granting some kind of spell immunity for a very brief moment after they get activated.

In this video you can see Sivir blocking 3 spells at once with her spell shield: Thresh's Q, Tristana's W and Tristana's E.