I've reported all such entries, as frankly they are a bad way to teach these things as they are not inherently obvious as to which they should be. One capitol letter and it's a sentence in and of itself, and thus they are easy to overlook. They should turn these into complete sentences to teach the grammatical item in question.
Ami is a relative pronoun which starts a relative clause. This task here is just a fragment of a sentence, like "Tartom, ami nagy és nehéz." - "I hold (the thing) that is big and heavy." Ami is related to mi - what.
A full sentence of "That (thing) is big and heavy." would translate to "Az nagy és nehéz". On the other hand, "Az a nagy és néhez" would translate to "That big and heavy thing" or "That is the big and heavy thing."
The lack of a verb makes these things a bit ... nehéz to figure out. :´)
Ami is related to the question word mi - what. As such it is a relative pronoun that refers to a general object and can also be translated as "what" or "that". A daru mindent emel, ami nagy és nehéz - The crane lifts everything that is big and heavy.
Aki is related to the question word ki - who. It's a relative pronoun that refers to people, usually also translated as "who". Azt a gyereket tartom, aki nagy és nehéz. - I am carrying the child who is big and heavy.