Dispositional attribution is a big, fancy term that means how we explain someone's behavior based on their personality or character.
Imagine you have a friend who always seems to be happy and cheerful. When you see her laughing and having fun, you might think that she is a happy person who likes to have a good time (that's dispositional attribution).
On the other hand, if you see her crying or upset, you might think that something bad must have happened to her (that's situational attribution, which is the opposite of dispositional attribution).
Basically, dispositional attribution is when we think that someone's behavior is caused by who they are as a person, while situational attribution is when we think their behavior is caused by something that happened to them.