Gmail let's the users add attachments in their email. The attachments can be dragged straight from the computer into the body of an email or selected from your Google Drive. But the email service has a limit for those attachment sizes. A limit was set to stop spammers from flooding the inbox and Google's servers with large, useless files.
Gmail and it's servers can only handle the attachments up to 25MB in size and so any attachment files above this size will be showing an error message.
If the user wants to attach files larger than 25MB, Google will start uploading the data to the user's Google Drive account. After that file is uploaded, it will place a link to those files in the email and lets your email's recipient access the files easily.
The user can also upload the files to Google Drive beforehand. The drive will then generate the link for the user so that they can send it to the directed person to access and so big files can be transferred.