EtherChannel enables bundling multiple physical links connecting same devices into a single logical link. I will try to show you how it is configured and how it works.
The issue with one uplink
I made an example with 8 clients connected to two Cisco 3850 switches. For start, those two switches are connected together with 1G copper on Interface Ge1/23. The clients are also connected to 1G ports. In this case, when all of those four clients on the left side start simultaneously sending traffic at full speed to different computers on the right side, they will congest the uplink between switches and some traffic will be dropped.