child.html

<template>
    <style>
        table tr td{
            padding: 10px;
            border: 1px solid black;
        }
    </style>

    <table style="background:white;">
        <tr>
            <td>
                Reactive Public Property
                <lightning-input type="text" onchange={handleChange1}></lightning-input>
                <br/>
                <b>Value: {reactivePublicProperty}</b>
            </td>
        </tr>
        <tr>
            <td>
                Reactive Private Property
                <lightning-input type="text" onchange={handleChange2}></lightning-input>
                <br/>
                <b>Value: {reactivePrivateProperty}</b>
            </td>
        </tr>
        <tr>
            <td>
                Non-Reactive Private Property
                <lightning-input type="text" onchange={handleChange3}></lightning-input>
                <br/>
                <b>Value: {nonReactivePrivateProperty}</b>
            </td>
        </tr>
    </table>
</template>

child.js

import { LightningElement,api, track } from 'lwc';

export default class Child extends LightningElement {
    @api reactivePublicProperty;
    @track reactivePrivateProperty;
    nonReactivePrivateProperty;

    handleChange1(event) {
        this.reactivePublicProperty = event.target.value;
    }

    handleChange2(event) {
        this.reactivePrivateProperty = event.target.value;
    }

    handleChange3(event) {
        this.nonReactivePrivateProperty = event.target.value;
    }
}

parent.html

<template>
    <style>
        table tr td{
            padding: 10px;
            border: 1px solid black;
        }
    </style>
    
    <table style="background: #e6efe2;">
        <tr>
            <td>
                <b>Passing value to reactive public property</b>
                <c-child reactive-public-property="Hi from parent - 1"></c-child>
            </td>
            <td>
                <b>Passing value to reactive private property</b>
                <c-child reactive-private-property="Hi from parent - 2"></c-child>
            </td>
            <td>
                <b>Passing value to non-reactive private property</b>
                <c-child non-reactive-private-property="Hi from parent - 3"></c-child>
            </td>
        </tr>
    </table>
</template>

parent.js-meta.xml

<?xml version="1.0" encoding="UTF-8"?>
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata" fqn="parent">
    <apiVersion>45.0</apiVersion>
    <isExposed>true</isExposed>
    <targets>
        <target>lightning__AppPage</target>
        <target>lightning__RecordPage</target>
        <target>lightning__HomePage</target>
    </targets>
</LightningComponentBundle>