Symmetrical internet is where your upload and download bandwidth matches.
For example: 500 Mbps download & 500 Mbps upload.
An asymmetrical internet connection does not match. Typically, the upload is significantly lower than the download.
For example: 500 Mbps download & 20 Mbps upload.
This can cause issues with various upload-heavy tasks, such as checking the cameras at your house while on your phone at the grocery store because the cameras have to upload their video stream from your internet connection to your phone. Another example is trying to send large files to cloud storage services (OneDrive, Google Drive, iCloud, Dropbox, etc.).