Default filters are the ones people got when they opened the watchlist the first time on the old configuration. You see all contributions.
You can redefined default filters by clearing all of them (trash bin icon), which will give you access to the "restaure default filters". It is possible to define a different set of filters for your wiki, after a community consensus.
You can personally set your favorite set of filters, by saving a preset and defining by default.
You can exclude some namespaces, for instance talk pages (click on "namespace", then select one or multiple namespaces, then click on "excluding namespaces"). That selection can be saved for you as well, so that you can switch to it anytime.