This is the 16846th most frequent Russian word.
собирающих
"Collecting" or "gathering" (as a present participle).
In this sentence, "собирающие" is used adjectively to describe the machines that are performing the action of gathering the harvest.
Машины, собирающие урожай, работают без остановки.
Machines that gather the harvest work without stopping.
Here, "собирающих" is modifying "людей" (people) and means those are carrying out the action of collecting garbage.
Я видел людей, собирающих мусор в парке.
I saw people collecting garbage in the park.
The word "собирающих" describes the people carrying out the activity of collecting books for the library.