The Expect Value (E) is the number of hits that a person would expect by chance when searching a database. The lower this value is, the higher the Score (S) of the match is. The closer the E-value gets to zero, the better the match is. You can increase or decrease it, depending on how small your sequences are, for example, if you are searching smaller sequences, you could increase the E value from 10 to 1000.
The lower the E-value, or the closer it is to zero, the more "significant" the match is. But another factor that affects the E-value is the length of the query. Since shorter ones have a higher chance of occurring in the database, they tend to have a higher E-value.