Learn Italian: Spero che il pasto non sia pesante, perché bisogna digerire bene. - I hope that the meal is not heavy, because it is necessary to digest well.

Sia is the present subjunctive form of essere (to be) for the third person singular (il pasto). We must use the subjunctive here because the sentence starts with spero che (I hope that). Verbs expressing hope, desire, or opinion usually trigger the subjunctive in the dependent clause.

Can we leave out the word che like we do in English when we say "I hope the meal..."?

No. In English, the word "that" is often optional, but in Italian, the conjunction che is strictly required to connect the main clause (spero) with the dependent subjunctive clause.

How exactly does bisogna work in this sentence?

Bisogna is an impersonal verb that means "it is necessary" or "one must." It does not take a standard subject like "I" or "you." Instead, it is always used in the third-person singular and is followed directly by an infinitive verb, which is digerire (to digest) here.

Is pesante used specifically for food here?

Yes, just like in English, pesante can mean physically heavy, but when talking about food or a meal (il pasto), it means "heavy" in the sense of being rich or hard to digest. The opposite of this would be leggero (light).