All subordinate clauses put their verb at the end. For example:
- subordinating conjunction clauses: "Er hilft mir, weil er freundlich ist"
- relative clauses: "Der Mann, der mir hilft, ist freundlich"
- indirect questions: "Ich weiß nicht, warum er mir hilft"
- indirect statements ("dass"): "Ich hoffe, dass er mir hilft"
This is a grammar issue. It's not about meaning. The comma is used in German exactly the same way as it is used in English. When a sentence has more than one clause, a comma goes between the clauses and before a connecting word: , which , that , so , because ,etc. The difference is that German culture is more strict about always using them. Since about 1890 English grammar has tended to opt out of commas unless they give clarity. English grammar has tended to opt out of commas, unless they give clarity. (with comma!)
It's kind of a sentence inside a sentence. It's a section of a sentence with its own verb separate from the main verb of the sentence. So in this sentence, the main verb is "hoffe"; the sentence is expressing that "I" am hoping something. Then there's the rest of the sentence, "dass er mir hilft," with a separate verb showing what "I" am hoping.
Some indicators for a subordinate clause are:
- a subordinating conjunction: "Er hilft mir, weil mein Koffer schwer ist" ("dass" fits into this category)
- a question word (was, wo, ob, etc.) in the middle of the sentence: "Er fragt mich, ob er mir helfen kann"
- "zu" with an infinitive verb: "Er versucht, mir zu helfen"
- a relative pronoun: "Er ist der Mann, der mir hilft"
You can read more here.