An Options Assignment is a situation where the option seller has to fulfil the obligation agreed upon in the options contract, by either selling or buying the underlying security at the strike price (also called exercise price).
If a call option is assigned (when ITM at expiration) the call option seller will have to sell the underlying security at the strike price.
On the other hand, if a put option is assigned, (when ITM at expiration) the put option seller will have to buy the underlying security at the strike price.
Example
