die Leute is more like "people/folks" and is often more conversational in tone.
die Menschen is more like "people/humans" and is often more neutral in tone.
You will find other explanations in the comments on other lessons. The best way to understand the difference will be to see it being used in context out in the world, though.
The things you need to consider: gender, case and article/determiner.
In this example, the gender is plural, case is nominative and the definite article (die) is used. Therefore, the adjective requires the -en ending.
I suggest you familiarise yourself with the rules for adjective endings here:
It will pay to keep reviewing them until they are internalised and feel natural.
If it's a preceded adjective ("die" comes before it), it's wichtigen. If it is unpreceded, then it is wichtige.
If it helps, all unpreceded adjectives take the ending that the definite article would normally have - in nominative, masculine takes -er, feminine -e, neuter -es, plural -e.