Learn Polish: Czy te duże ryby jedzą mięso?

The singular word for fish is ryba, which is a feminine noun. In the nominative plural, feminine nouns usually change their ending from -a to -y, giving us ryby. Since fish are animals, they fall into the non-virile (not male human) category.

Why do the words for these and big end in -e (te duże)?

Adjectives and pointing words must match the noun they describe. For non-virile plural nouns like ryby, the word ten (this) changes to te (these), and the adjective duży (big) takes the -e ending to become duże.

Jedzą means they eat, but what is the dictionary form of this verb?

The dictionary form is jeść. It is a very irregular verb in Polish. Jedzą is the they form, and it is best to simply memorize it because it does not follow the standard conjugation rules you have learned so far.

Why doesn't the word mięso change its ending here? Isn't it the object of the sentence?

You are right that mięso (meat) is the direct object, which means it is in the accusative case. However, mięso is a neuter noun because it ends in -o. For all neuter nouns, the accusative case looks exactly the same as the basic dictionary form.