What is a Lightning channel?
A channel is a funded link between two Lightning nodes. To open one you make a single on-chain Bitcoin transaction that locks a balance into the channel. After that, the two nodes can send payments back and forth across it as often as they like, instantly and for almost nothing, without writing anything to the blockchain. When you are finally done, closing the channel settles the final balance back on-chain. The blockchain sees the open and the close; everything in between stays off it, and that is where Lightning’s speed comes from.
You almost never have a direct channel with everyone you want to pay. The network solves this by routing: a payment hops through a chain of connected channels, node to node, until it reaches the destination. Each hop just shifts balance along a channel that already exists.
Why do channels need managing?
Because a channel has a direction. The funds in it sit on one side or the other, which gives it outbound capacity, what you can send, and inbound capacity, what you can receive. Send a lot and your balance shifts to the far side, leaving you plenty of room to receive but little to send. Receive a lot and the reverse happens. Neither is a fault; it is just how the balance moves.
This is the upkeep that running your own node adds. You open channels with enough capacity for what you do, keep an eye on the inbound and outbound balance, and occasionally rebalance or open another. It is the price of holding your own funds instead of letting a custodial app paper over all of it for you.