If you have many similar filter options, let's say sizes 10, 10.5, 10/11, and Large, you can merge filters in the way that only size '10' option appears in the filter menu, and once this option is selected products with sizes 10.5, 10/11, and Large will appear too.
To merge filters, click the '+' icon next to the filter option and then select filters/tags you want to merge into it.
Note, in the window where you select filters to merge, there is 'Hide the matching filters' at the bottom - this should be checked if you want merged filters (sizes 10.5, 10/11, Large etc.) to be hidden.
Here is the video guide on merging filters:
Another example how to merge colors into primary colors. This is much the same as the sizes, above. You may, however, want to create new filters for each of your 'primary' colors, if they do not exist yet. Press 'Add Filters', then select 'Tags', and type in your primary color names. You can then merge your filters into this group.