Your Client ID is a unique number (xxx-xxx-xxxx) used to identify your AdWords account. With your ID you can easily link your account to other Google products such as Analytics, My Business, etc.
Keep in mind, that everyone who has your ID can use it to access your AdWords information, so keep it safely!
To find your ID follow the next steps:
1) Sign up to Google Adwords. Go to https://www.google.com/intl/en/adwords/
Here you can Start now – to create a new account or Sign in, if you already use Google Adwords.
2) Once you sign in, your Client ID will be at the top right corner of any AdWords page, just above your email address.