Electronic signatures are ways to sign digital documents, just like how you would sign a physical paper. They come in three types:
Scribble Electronic Signature (SES): An electronic signature is defined as "data in electronic form which is attached to or logically associated with other data in electronic form and which is used by the signatory to sign". Thus, something as simple as writing your name under an e-mail might constitute an electronic signature.
Advanced Electronic Signature (AES): This one is a bit more complex. It's a unique signature that can identify you as the person who signed the document. It's created in a way that lets you keep control over it. Also, it's connected to the document so that if anyone tries to change something after you've signed it, those changes can be spotted.
Qualified Electronic Signature (QES): This is the most secure type of electronic signature. On top of having all the features of an advanced electronic signature, it's also made using a special device and is based on something called a "qualified certificate". This makes it the safest and most reliable way to sign something digitally, as it's very hard for anyone else to fake or misuse it.