An ice shelf is like a big floating block of ice that is attached to land on one side, but it sticks out into the water on the other side. It's kind of like how an ice cube floats in a glass of water, except on a much bigger scale.
Imagine you're building a sandcastle on the beach. When the tide comes in, the water reaches your castle and starts to surround it. Now, imagine your sandcastle is made of ice instead of sand. That's kind of what happens with ice shelves - they stick out into the water and get surrounded by it, but they're made of solid ice instead of sand.
These ice shelves are really important because they help keep the ice on land from melting too quickly. The big floating ice shelf acts as a barrier that slows down the flow of ice from land into the water. Without these ice shelves, the melting of large ice sheets and glaciers on land would go much faster, and that could cause sea levels to rise, which would be bad news for people who live near the coast.
Unfortunately, some ice shelves are starting to break apart due to climate change. When that happens, it can cause a chain reaction that leads to even more melting of ice on land. Scientists are working hard to figure out how to slow this down and prevent it from happening as much as possible.