Discohook is a popular tool for creating Discord embed messages using Webhooks. Sadly it's not perfect ... For many features an additional bot is needed and the setup can be confusing. Additionally the uptime has been far from perfect recently.
TLDR: Embed Generator is the superior alternative to Discohook in terms of features, usability, and stability.
Discord Webhooks are a powerful feature which allows Discord server administrators to generate a unique URL which can be used by third party services to send messages to a Discord channel.
Discord is not the only platform that supports webhooks. They are commonly used by platforms to receive updates and notifications from third party services that integrate with them. These third party services just have to know the webhook URL and the format that is expected to send messages programmatically.
Webhooks on Discord are unique in the sense that they are not only used programmatically by third party services to send updates. Many people use Discord webhooks to send out messages by hand instead of sending them with their own Discord account because webhook messages have way more formatting options than normal messages.
This is why there are tools like Embed Generator that can help you sending messages through Discord webhooks.