Server-side validation is an imperative security layer for web applications. It involves validating user inputs on the server-side before processing them, thus ensuring data integrity and preventing potential security threats like SQL injection and Cross-Site Scripting (XSS) attacks.

In the context of HTML forms, server-side validation is implemented using server-side scripting languages such as Python, PHP, or Node.js. For instance, in a Python Flask application, you can use the WTForms library to validate form inputs. WTForms allows you to define form classes and apply validators to form fields, which are functions checking if the data in a field meets certain conditions.

Here's an example of how server-side validation can be implemented in a Python Flask application:

from flask import Flask, request, render_template
from wtforms import Form, StringField, PasswordField, validators

app = Flask(__name__)

class RegistrationForm(Form):
    email = StringField('Email', [validators.Length(min=6, max=35), validators.Email()])
    password = PasswordField('Password', [validators.Length(min=8)])

@app.route('/register', methods=['GET', 'POST'])
def register():
    form = RegistrationForm(request.form)
    if request.method == 'POST' and form.validate():
        # Save the user data to the database
    else:
        # Render the registration form with error messages
        return render_template('register.html', form=form)

In this code, a RegistrationForm class is defined with two fields: email and password. The email field has two validators: Length and Email. The Length validator ensures that the email is between 6 and 35 characters long, and the Email validator ensures that it's a valid email format. The password field has a Length validator that ensures it's at least 8 characters long.

When the /register route is accessed with a POST request, the form data is validated. If the data is valid, it's processed (e.g., saved to the database). If the data is invalid, the registration form is rendered again with error messages.

It’s important to note that server-side validation should not replace client-side validation but complement it. Server-side validation provides a second line of defense against invalid or malicious data, especially in cases where client-side validation is bypassed or fails.