Learn German: Obwohl die Ameise winzig ist, ist sie bedeutend stärker als der Mensch. - Although the ant is tiny, it is considerably stronger than the human.

Bedeutend (meaning "significantly" or "considerably") is used here as a degree adverb to intensify the comparative adjective stärker. While at lower levels you would likely just use viel (viel stärker = much stronger), using words like bedeutend, wesentlich, or weitaus elevates your German and sounds much more precise.

Why does the second half of the sentence start with the verb (ist sie) instead of the subject (sie ist)?

This is due to the standard German verb-second (V2) rule. The entire subordinate clause (Obwohl die Ameise winzig ist) occupies the first position of the overall sentence. Therefore, the conjugated verb of the main clause (ist) must immediately follow the comma to take the second position, pushing the subject (sie) right after it.

Why is it als der Mensch in the nominative case instead of the accusative or dative?

When you use als (than) to compare two things, the noun after als takes the exact same case as the noun it is being compared to. Here, the ant (die Ameise / sie) is the subject of the sentence, so it is in the nominative case. Therefore, der Mensch must also be in the nominative.

Is bedeutend related to the verb ?