What does event.body?

I am new and I would like to find someone to help me understand the two lines explained below. In particular I cannot understand what event.body returns in this line:

let requestJSON = JSON.parse (event.body);

and what returns JSON.stringify (body) in this line:

body = JSON.stringify (body).

Can anyone kindly help me understand?

const AWS = require("aws-sdk");

const dynamo = new AWS.DynamoDB.DocumentClient();

exports.handler = async (event, context) => {
    let body;
    let statusCode = 200;
    const headers = {
        "Content-Type": "application/json"
    };

    try {
        switch (event.routeKey) {
            case "PUT /items":
                let requestJSON = JSON.parse(event.body);
                await dynamo
                    .put({
                        TableName: "http-crud-tutorial-items",
                        Item: {
                            id: requestJSON.id,
                            price: requestJSON.price,
                            name: requestJSON.name
                        }
                    })
                    .promise();
                body = `Put item ${requestJSON.id}`;
                break;
            default:
                throw new Error(`Unsupported route: "${event.routeKey}"`);
        }
    } catch (err) {
        statusCode = 400;
        body = err.message;
    } finally {
        body = JSON.stringify(body);
    }

    return {
        statusCode,
        body,
        headers
    };
};```


Obviously I refer to the example shown.