This is a clear glass filter that has no optical function at all but is simply designed to protect the front element of the lens from dust, smears, scratches and moisture, even impacts. Protection filters screw directly into the lens’s filter thread, so you will need to get one the right size. You can also use UV filters as protection filters, although the UV filtering is hardly relevant today.
Not every photographer thinks they are necessary. Protection filters are not especially cheap because they have to be of sufficient optical quality that they don’t affect the image quality captured, and if you do break one, replacing it won’t be cheap. It’s also likely that any impact strong enough to break the filter has done other damage to the camera or lens besides.